If your company is currently using ISDN or traditional landlines, now is the time to change. BT plans to complete its PSTN and ISDN switch-off by 31st January 2027, which means that every UK business using legacy phone lines will need to plan for a move over to a digital solution, and SIP trunking is the suggested migration path.
In the UK, SIP trunk providers allow you to take your existing PBX phone system and migrate your voice calls across the internet. You improve call quality, reduce recurring expenses every month, and can simply scale channels up or down when your business requirements change.
However, not all SIP trunk providers are the same. Since every provider is different in terms of pricing structures, compatibility with PBX systems, whether or not they have UK numbers available, and other factors, finding the ideal VoIP solution can take quite a bit of time.
In this guide, ComparedBusiness UK has ranked the top 9 SIP trunk providers in the UK for comparison in 2026. Whether you’re looking for pricing, essential features, a key limitations breakdown, or a straightforward verdict, we have you covered.
Key takeaways
BT’s PSTN/ISDN switch-off deadline is 31st January 2027, so UK businesses must migrate now or risk losing phone service.
SIP trunking is the most cost-effective ISDN replacement for businesses with an existing PBX system.
The VoIP Shop is the top pick for UK SMEs because of its transparent pricing, UK-based support, and easy onboarding.
Vonage is well-suited for enterprises and developer-led teams that need elastic SIP trunking with API programmability.
BT Cloud Voice is the safest migration route for existing BT customers, though long contracts can be a drawback.
RingCentral is best for mid-to-large businesses that want SIP trunking alongside a full UCaaS platform.
8×8 is a strong enterprise choice for businesses needing unified communications with international calling to 48+ countries.
Gamma is the UK’s largest independent SIP trunk carrier, but it sells exclusively through resellers rather than directly to businesses.
Dial 9 is the budget pick for small UK businesses running their own PBX, with trunks from just £3/month.
UK SIP trunk channel costs range from approximately £3 to £4.95 per channel per month, depending on the provider and plan.
What Is SIP Trunking?
SIP trunking, also known as Session Initiation Protocol trunking, is a technology that allows us to connect your traditional on-premises PBX (private branch exchange) to the public switched telephone network (PSTN) over the internet.
A SIP trunk provides a virtual connection in place of physical copper ISDN lines, so your office calls run over your broadband connection rather than dedicated cables.
Well, to put it simply, your PBX is in charge of all the things that happen within your office: extensions, call routing, voicemail, and so on. It is always connected to the outside world over ISDN lines running from the exchange.
A SIP trunk performs precisely the same function, but through a digital internet-based connection. You retain your desk phones, your existing PBX, and the way you typically work, with one difference: a virtual cable instead of a physical one.
SIP Trunking Vs Hosted VoIP: What’s the Difference?
SIP trunking is not a hosted VoIP or cloud phone system. With hosted VoIP, your provider manages everything, including the PBX in someone else’s cloud. Conversely, SIP trunking allows you to keep your own PBX on site with just the exterior ISDN lines swapped for a SIP connection.
SIP trunking is the best value option for companies that have an existing PBX and do not wish to replace it. If you are starting entirely from scratch or desire a cloud-based solution, hosted VoIP may be the ideal way to go about it.
Get Top SIP Trunk Providers Quotes Today
How We Evaluated The Top SIP Trunk Providers In The UK?
Each of the SIP trunk providers on this list has been rated against six factors that matter most to UK businesses in 2026. Here is what we looked at:
| Evaluation Criteria | What We Looked At |
|---|---|
|
Call Quality and Uptime |
SLA guarantees, redundancy in the UK Tier 1 network, and automatic router failover during outages. |
|
Pricing Transparency |
Per-channel pricing, available minutes, one-off charges, and contract options. |
|
Availability Of UK Numbers |
Geographic numbers (01/02/03), non-geographic, and 07 mobile number support. |
|
PBX and Hardware Compatibility |
Compatibility with 3CX, Asterisk, FreePBX, Yealink, Cisco, and Microsoft Teams Direct Routing. |
|
Ofcom Compliance and UK GDPR |
Regulatory obligations with respect to UK operators and data processing. |
|
Support Quality |
Support teams are based in the UK, with a commitment to SLA response times and onboarding assistance. |
We only evaluated providers that deliver local or location-aware support for the UK market and provide UK geographic numbers and clear PSTN switch-off migration paths. Pricing data is based on published rates and confirmed source information, as of June 2026.
Top 9 SIP Trunk Providers In The UK: A Quick Overview
Here is a quick overview of the top 9 SIP trunk providers in the UK:
| Provider | Starting Price | PBX-Compatible | Contract | Support | Best For |
|---|---|---|---|---|---|
|
The VoIP Shop |
Custom Quote |
3CX, Asterisk, and FreePBX |
Monthly rolling |
UK-based |
UK SMEs and startups |
|
Vonage |
Custom Pricing |
SIP/API-compatible PBX |
Annual flexible |
Global 24/7 |
Enterprise and developer teams |
|
BT Cloud Voice |
Contact The Sales Team |
BT-compatible PBX |
60 months |
UK-based 24/7 |
Small, medium and large existing BT customers |
|
RingCentral |
Custom Pricing |
IP PBX and MS Teams |
Annual |
Global 24/7 |
Medium to large businesses |
|
8x8 |
Quote-Based Pricing |
UCaaS/IP PBX |
Annual |
Global 24/7 |
Enterprise and contact centres |
|
Gamma |
Custom Pricing |
Yes |
SBCs, iPBX, and MS Teams |
Varies |
Resellers and their customers |
|
Dial 9 |
Starts from £3/month |
Yes |
PBX |
No minimum term |
Budget-conscious small businesses |
|
Sipgate |
Starts from £4.95/month |
Yes |
SIP-enabled PBX |
Monthly, no setup fee |
Cost-conscious tech-savvy SMEs |
|
Spitfire |
Starts from £4/month |
Yes |
PBX and MS Teams |
Flexible |
Businesses looking for call quality |
- Spitfire SIP trunking starts from £4/month.
- Sipgate SIP trunking starts from £4.95/month.
- Dial 9 SIP trunking starts from £3/month.
For UK carriers that publish per-channel trunks, rates vary from £3 to £4.95/channel/month.
Now, let’s take a look at the details of each SIP trunk provider:
1. The VoIP Shop
Model: UK-Native Cloud SIP Trunking and Hosted PBX Provider | Location: Leicester, UK
Overview:
The VoIP Shop offers cloud-based SIP trunking and hosted phone systems to small and medium-sized businesses in the UK. Based in Leicester, The VoIP Shop provides simple SIP trunking on a carrier-grade network with UK-based support, serving over 10,000 UK businesses across the legal, healthcare, finance, and education sectors.
They have fast set-up and transparent pricing, so they are usually the first choice for SMEs that are moving from ISDN before the 2027 switch-off. Alongside their SIP trunk services, The VoIP Shop also offers AI-driven answering services, broadband and CRM integrations.
Best For:
UK-based small and medium enterprises or startups looking for a low-cost, easy-to-onboard SIP trunking solution with local support.
Pricing:
SIP trunking pricing is determined by the number of channels and how much they will be used. Contact their team for a custom quote. No long lock-in contracts, with monthly rolling options.
Key Strengths:
- UK support team with lots of great customer reviews from verified sources.
- Compatible with 3CX, Asterisk, FreePBX, and most SIP-enabled hardware.
- With quick onboarding, companies have gone live with low technical complexity.
- No hidden setup fees on standard plans with flexible and transparent pricing.
Limitations:
- Works on a smaller scale than a national carrier like BT or an international platform like RingCentral.
- No clear pricing information is available online; you need to request a quote for SIP trunk services.
- It is not appropriate for huge enterprises requiring carrier-level SLA guarantees.
Third-Party Rating:
Rated ‘Excellent’ based on verified customer reviews on Trustpilot. Consistent praise for the support response and the smooth porting of numbers.
Verdict:
The VoIP Shop is our number one recommendation for UK SMEs wanting a straightforward, low-cost SIP trunking solution supported by genuine local support.
2. Vonage
Model: Global Elastic SIP Trunking with API-first Architecture | Location: European HQ in Shoreditch, London, UK (company founded in the US).
Overview:
Vonage, acquired by Ericsson in 2022 for $6.2 billion, provides enterprise-grade elastic SIP trunking and global coverage across more than 140 countries. Vonage is registered in the UK as Vonage Limited and Vonage Business Limited, with their European headquarters located in Shoreditch, London.
The platform leverages the Vonage Communications APIs, making it a developer-first product that allows for instant, demand-driven scalability of SIP trunk channels without having to manually provision these resources individually.
With Vonage, the best collaboration tools and third-party integrations come built-in, including more than 25 CRM platforms, plus unlimited international calling offered in some plans.
Best For:
Enterprises and developer-led teams that want API-programmable SIP trunks with worldwide number coverage.
Pricing:
Quote-based pricing.
Key Strengths:
- Elastic SIP trunking allows on-demand scalability of channels via the API, without the need for capacity planning.
- Worldwide number coverage in over 140 countries, along with UK DDI and non-geographic numbers.
- Per-second billing saves money for businesses with variable call volumes.
- It offers 25+ CRM and business tools for a deep integration ecosystem.
Limitations:
- It is focused on developers, so technical knowledge is required to configure and optimise.
- Without diligent plan management, add-on features can make monthly bills unpredictable.
- Customer service quality has received mixed reviews in third-party assessments.
Third-Party Rating:
As of June 2026, 4.7/5 on Capterra (831+ reviews). The G2 score reflects that the Vonage Business platform is quite enterprise-oriented.
Verdict:
Vonage is ideal for technologically savvy businesses and enterprises that require flexible, globally expandable SIP trunking with API access.
3. BT Cloud Voice
Model: UK National Carrier of Managed Cloud Voice and SIP Trunking | Location: London, UK
Overview:
BT Cloud Voice is BT UK’s managed business VoIP and SIP trunking solution. Backed by the UK’s most advanced network infrastructure and complete BT end-to-end routing, it provides a fully managed alternative to ISDN for businesses.
BT Cloud Voice particularly targets organisations that are already BT customers and want a single provider for their connectivity, phone and voice services.
As the UK PLC that pioneered the PSTN switch-off programme, BT is an authority when it comes to ISDN migration pathways, and its Cloud Voice product is directly marketed as the recommended ISDN replacement for businesses in the UK.
Best For:
BT customers with existing contracts and multi-site businesses in the UK that require an all-in-one managed ISDN to digital voice migration.
Pricing:
Pricing is negotiated directly with BT or an authorised partner.
Key Strengths:
- With unrivalled UK infrastructure, it offers an end-to-end network for consistent, reliable call quality.
- PSTN switch-off authority. BT understands the national migration programme better than anyone else, as it is leading the programme.
- Core trunking plans include disaster recovery and failover routing at no extra cost.
- One provider for connectivity, phones, mobiles, and voice, all in one contract.
Limitations:
- Extensive contract periods: BT Cloud Voice needs agreements of up to 60-month (5-year) commitments, the longest lock-in in the industry.
- Annual CPI+3.9% price escalators lead to annual cost increases.
- As of June 2026, BT Cloud Voice has a 3.7 Trustpilot rating, reflecting numerous complaints about billing and customer service turnaround times.
Third-Party Rating:
- 3.7/5 based on Trustpilot (13,770+ reviews). As of June 2026.
- Key Note: BT’s business services rate better than its consumer division, but the problems with billing and the resolution of complaints are constant.
Verdict:
If you’re an existing BT customer moving from ISDN, then BT Cloud Voice has the least resistance possible, but anyone concerned about 5-year lock-ins should look at independent alternatives first.
4. RingCentral
Model: Enterprise UCaaS Platform with Integrated SIP Trunking | Location: London, UK, with a headquarters in the US.
Overview:
With over 400,000 business customers globally, RingCentral is one of the most deployed unified communications platforms in the market.
In the UK, it runs through its RingEX platform, a single product for cloud communication solutions that offers calling, messaging, video conferencing and SIP trunking.
Targeted at organisations that need SIP channels to supplement a larger UCaaS offering, RingCentral’s SIP platform works with both premise-based PBX integration and Microsoft Teams direct routing.
It is designed for businesses that need robust corporate communications across a range of channels. It offers a 99.99% uptime SLA and integration with 500+ apps, including Salesforce, HubSpot, and Microsoft 365.
Best For:
Mid-to-large UK enterprises that require SIP trunking within a fully integrated UCaaS stack, particularly businesses using Microsoft Teams or Salesforce.
Pricing:
Enterprise deployments are quote-based, and the channel pricing is specific to the SIP Trunk.
Key Strengths:
- 99.99% uptime SLA, the highest availability guarantee in the market.
- Microsoft Teams Direct Routing support is available for clients who use Teams as their softphone.
- 500+ integrations, including Salesforce, HubSpot, and Microsoft 365.
- Unified platform for voice, video, messaging and SIP all in a single admin console.
Limitations:
- The Trustpilot score is around 2.2/5; issues with customer service and contract exit are common grievances.
- Monthly consumption charges can become pricey due to a working model based on an annual contract with rigorous billing.
- This service is not a great match for SMEs that only need simple SIP trunking and do not require the broader UCaaS feature set.
Third-Party Rating:
- G2: 4.2/5 (900+ reviews).
- Trustpilot: 2.2/5 (1,800+ reviews). As of June 2026.
Verdict:
RingCentral is best for businesses that need a complete UCaaS ecosystem with SIP trunking. Smaller companies should consider smaller solutions.
5. 8x8 Work
Model: Enterprise Cloud Communication and UCaaS Integrated SIP Trunking | Location: London, UK
Overview:
8×8 is a cloud communications platform that provides SIP trunking deeply integrated with its complete UCaaS solution.
Given their global reach and unlimited calling to 48+ countries on eligible plans, 8×8 is a great fit for enterprises with multi-country operations or high international call volumes.
8×8 offers complete local number portability, TLS/SRTP encryption and an easy-to-navigate administration portal. Their SIP trunking integrates with their larger platform, which translates to businesses receiving HD voice, auto-attendant, call recording and analytics together with the trunking service that is fundamental.
Pricing and complexity of functionality make it a better fit for enterprises rather than SMEs.
Best For:
Best for enterprise and mid-market UK businesses that require integrated SIP trunking with UCaaS capabilities, international calling, and compliance-grade security.
Pricing:
Enterprise and contact centre plans are available on a quote basis. While it offers a range of UCaaS solutions, the best value comes from using the full 8×8 UCaaS platform.
Key Strengths:
- Unlimited calling to 48+ countries, ideal for international businesses.
- Built-in fraud prevention tools using the TLS/SRTP encryption.
- UCaaS integrated SIP trunking offers HD voice, analytics and call recording in a single solution.
- The easy-to-administer portal requires no manual redundancy configuration.
Limitations:
- Launch price higher than most UK-native SIP trunk providers, not the most cost-effective option for basic trunking-only requirements.
- The bundle UCaaS model means you pay for what you don’t need.
- It can be complex for small businesses with limited technical knowledge.
Third-Party Rating:
- G2: 4.1/5 score based on enterprise-verified reviews. As of June 2026.
Verdict:
8×8 is the best suited for UK enterprise buyers with global operations that are interested in a premium communications platform as a service, but not just a SIP trunk.
Want to find the VoIP providers? See our guides:
6. Gamma
Model: Wholesale SIP Trunk Carrier and Network Infrastructure Provider | Location: London, UK
Overview:
Gamma is one of the largest independent telecoms carriers in the UK, delivering SIP trunking and unified communications technology that supports phone services for about a third of UK businesses.
Gamma is building a nationwide fibre network by taking advantage of existing infrastructure and selling directly in the areas it serves. However, most end customers purchase their network and SIP trunking platform through a channel of several thousand resellers, managed service providers, and IT partners, who then bundle the service for resale under their branding.
Best For:
Large organisations purchasing SIP trunking through a reseller or managed service provider need robust, carrier-grade infrastructure in the background.
Pricing:
Quote-based pricing
Key Strengths:
- The largest independent SIP carrier in the UK, they support many resellers and managed service providers.
- It supports business continuity through a resilient mesh and active/standby during failover.
- Support for Microsoft Teams Direct Routing natively for businesses adopting Teams Phone as their standard.
- Call routing, reporting, and IVR control are available, with an optional SIP Trunk Call Manager portal.
Limitations:
- Not directly sold to many businesses; hence, pricing, support quality, and contract terms are solely dependent on your reseller.
- Similar product comparisons are more difficult for buyers due to the nondisclosure of pricing information.
- Customer reviews can be inconsistent and vary significantly depending on the partner who provides the service.
Third-Party Rating:
The Trustpilot rating is 2.9/5, but this varies because most reviewers are describing resellers, not Gamma.
Verdict:
Ideal for businesses that already have, or are comfortable working through, a partner reseller and need carrier-level infrastructure below their phone system.
7. Dial 9
Model: Direct-to-Business SIP Trunk Provider | Location: Poole, Dorset, UK
Overview:
Founded in 2011, Dial 9 is a UK communications provider that offers professional and cost-effective telephony solutions for small and medium-sized businesses.
Its SIP trunking service allows businesses that have their own PBX or phone server to connect directly to the public telephone network, and the setup is designed so as not to take more than a few minutes via an online portal.
Dial 9 Connect is the company’s own internal platform, maintained by its engineers. Dial 9 offers a more stripped-down SIP trunk with entry-level pricing, flexible channel scaling, and no long-term contract requirements, so it is perfect for smaller businesses or first-time SIP trunking testers.
Best For:
Small businesses and tech-savvy users with their own PBX who prefer a low-cost, no-commitment, and easy-to-set-up SIP trunk.
Pricing:
Starting at £4.50/trunk/month for 4 SIP channels.
Key Strengths:
- Low entry price with a clear and simple cost breakdown.
- There are no contracts or minimum terms, allowing companies to progressively increase channels and retract them at their discomfort.
- Self-service via web portal, so you can get started in minutes.
- Customer support is based in the UK and is rated highly by customers.
Limitations:
- No advanced features like built-in call recording or analytics that larger carriers provide.
- As a smaller company, uptime guarantees and service-level commitments are published to a degree.
- It is designed for simpler deployments, not for complex enterprise multi-site environments.
Third-Party Rating:
Trustpilot: 4.7/5 (31 verified reviews).
Verdict:
Ideal for small businesses and those short on funds looking to get their SIP trunking needs met affordably, with no long contract lock-in.
8. Sipgate
Model: SIP Trunk Provider | Location: London, UK
Overview:
Sipgate delivers cloud telephony and SIP trunking solutions designed for every business type, from start-ups to multinational enterprise clients, across both the UK and European marketplaces.
The Sipgate trunking service links a VoIP-enabled PBX to the national telephone network, claiming no setup fees and rolling monthly contracts rather than long-term agreements.
It has a clear, published per-minute call-pricing service with plan tiers based on how many incoming lines a business requires, though outbound capacity is generous across the board. It also offers a 30-day free trial, allowing businesses to test SIP trunking in a relatively risk-free manner.
Best For:
Innovative businesses and technical teams who want transparent, pay-as-you-go pricing with no long contract lock-in.
Pricing:
Starts from £4.95/user/month.
Key Strengths:
- Transparent per-minute pricing that is straightforward in comparison to plans offered by bundled competitors.
- No setup fees, no minimum contract term or customer lock-in, which makes it possible to upgrade or downgrade at any time.
- 30-day free trial with test numbers and inclusive minutes.
- Integrates with Sipgate’s own hosted PBX and CRM tools for businesses wanting to expand later.
Limitations:
- Limited call quality relative to competitors, according to independent reviewers.
- Claims at least 98% SIP server availability over a year, lower than some competitors’ published uptime.
- Higher-tier incoming lines can be costly for companies with high inbound call volumes.
Third-Party Rating:
Capterra: 3/5 from only a couple of reviews.
Verdict:
It is best for companies that prefer pricing visibility and flexibility over a full-blown enterprise feature set.
9. Spitfire
Model: Network-Owning SIP Trunk Provider | Location: London, UK
Overview:
Spitfire is a UK-licensed telephone operator that has supplied SIP trunking and hosted telephony solutions for over 10 years. While many providers route your calls over third-party infrastructure, Spitfire owns and runs its own voice and data core network with two geographically separate SIP nodes (one in London, one in Manchester) for outbound redundancy.
This means it can provide prioritised call quality when supplying SIP trunks alongside its own Ethernet or broadband circuits. Spitfire also offers a SIP Gateway, which enables ISDN PBX systems to be connected to SIP trunks; it doesn’t require new phone hardware and could be particularly useful as the UK’s ISDN switch-off approaches.
Best For:
Businesses that want control of the quality of calls end-to-end and are willing to bundle SIP trunks with Spitfire’s own data connectivity.
Pricing:
Starts at £4/channel/month.
Key Strengths:
- Spitfire has its own UK voice and data network, enabling it to have direct control over call quality rather than naively depending on third parties.
- The SIP Gateway Product enables a migration of legacy ISDN PBX to SIP without hardware upgrades.
- Built-in outbound resilience and failover with dual geographically diverse network nodes.
- Supports Microsoft Teams Direct Routing in addition to conventional PBX connectivity.
Limitations:
- Spitfire provides a wide array of services in addition to SIP trunking, making it less specialised than pure SIP providers.
- Optimising call quality requires using Spitfire’s own connectivity, which may not suit all organisations that already have an established internet provider.
- Published UK numbers and contract details are less detailed compared to some larger competitors.
Third-Party Rating:
Trustpilot: 4.2/5 (130 reviews)
Verdict:
A good option for businesses that prioritise call quality and resilience, particularly if they are ready to take both connectivity and SIP trunking from the same provider.
The PSTN Switch-Off and Why SIP Trunking Matters in 2026
In the UK, the Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN) are being retired in a phased process, with a final deadline of 31st January 2027.
If your business relies on these traditional landline services, you must migrate before 31st January 2027 or risk losing your phone numbers and voice services.
What Does This Mean For Your Business?
- Any PBX located on-premises, especially those connected to ISDN lines, would be rendered inoperable after the switch-off is complete.
- Moving numbers from ISDN to SIP trunking normally takes 1 to 4 weeks, depending on the vendor. Plan early to prevent a crisis.
- For companies that wish to maintain their PBX, SIP trunking is the recommended alternative to ISDN.
- A transition from ISDN to SIP trunking offers savings on monthly line rental costs for businesses.
Ofcom is working with industry partners to support the migration programme. For businesses in regulated sectors such as healthcare, financial services and legal, it is vital that any SIP trunk provider they choose provides guarantees to comply with UK regulatory requirements and all aspects of its data-handling obligations under the terms of the UK GDPR.
Find The Top SIP Trunk Providers With ComparedBusiness UK
ComparedBusiness UK is the easiest way to compare leading SIP trunk providers side-by-side, including pricing, contract term, PBX compatibility and service quality.
If you’re an SME taking your initial steps into SIP trunking or a multi-site enterprise replacing ISDN at several sites, our comparison tools provide you with the information you need to choose the right SIP trunk provider.
Want to find the ideal SIP trunk provider for your business? Just fill out our quick quote form, and we will connect you with the top SIP trunk providers.
FAQs
Choosing a suitable SIP trunk provider will entirely depend on your business size and requirements. For UK SMEs, The VoIP Shop is the most cost-effective solution, as it offers affordable pricing, UK support and easy onboarding.
RingCentral and 8×8 both offer more robust UCaaS-integrated possibilities for enterprises, and Vonage is the best option for developer-led teams needing API flexibility.
UK SIP trunk prices range from £3 to £4.95 per channel per month, depending on the provider and plan type. Unlimited UK call bundles are mostly between £7 and £15/month. International calling and full UCaaS features start at £19/user/month.
Make sure to check the pricing for setup fees and number porting, as headline prices mostly omit them.
Yes. You can port your current UK geographic (01/02/03) and non-geographic numbers over to a SIP trunk carrier. Porting a phone number is usually completed within 1 to 4 weeks, depending on the provider and the complexity of your number range.
Most SIP trunk providers will handle this process on your behalf but may charge a minimal porting fee.
One simultaneous call is supported per channel. If a business has to handle 10 calls at the same time, then it requires 10 channels. To calculate your base start, measure the number of your maximum concurrent calls and increase by 20%. Most providers let you scale channels, so the number doesn’t need to be exact from day one.
Yes. SIP trunking is covered by Ofcom’s general authorisation framework, as set out in the Communications Act 2003.
UK providers must also meet key operational requirements, including emergency call handling, number portability, and UK data protection rules under the Privacy and Electronic Communications Regulations (PECR). They are also required to comply with UK GDPR standards, which align with the former EU GDPR framework.
As part of your SIP provider evaluation, ensure your provider is registered with Ofcom and has the necessary authorisations.
In most cases, yes. SIP trunking is compatible with virtually every SIP-enabled PBX, like 3CX, Asterisk, FreePBX or Yealink-based installs. Nonetheless, such a setup is not guaranteed to work, as it purely depends on your configurations.
The best SIP trunk providers in the UK offer published interoperability lists, and many, especially those with 3CX Titanium or Gold Partner status, make it even easier to provision a new installation by providing ready-made configuration templates.
