5 Best SIP Trunk Providers UK for Global Enterprises (2025)
Traditional phone systems are often costly, inflexible, and insecure. As the UK's 2025 Public Switched Telephone Network/Integrated Services Digital Network (PSTN/ISDN) switch-off approaches, finding a dependable Session Initiation Protocol (SIP) trunk provider is critical.
This modern solution provides international coverage, enhances call quality, and bolsters security, equipping businesses to handle today's communication demands.
The top UK SIP trunk providers for global enterprises are listed in this guide, emphasizing those offering scalable, compliant, and smoothly integrated services.
What is SIP trunking?
SIP trunking is a communications service that connects your business telephone system to the public telephone network using the internet instead of traditional phone lines or landlines.
SIP trunking:
- Replaces physical phone lines with virtual connections
- Transmits voice calls as data packets over your internet connection
- Works with most Internet Protocol Private Branch Exchange (IP PBX) systems and unified communications (UC) platforms
- Supports voice, video, and messaging services through a single connection
- Helps businesses transition from traditional UK landlines to more advanced solutions in preparation for the UK's 2025 PSTN switch-off.
Related: A Cloud Telephony Guide and 7 Top Providers (2025)
How does SIP trunking work?
SIP trunking directly connects your business and your service provider through the internet. Here's how it works:
- Your IP PBX system connects to the SIP trunk provider via the internet.
- The SIP provider routes phone calls between your system and the public telephone network.
- Outbound calls travel as data packets to the provider.
- The provider converts these packets to a format the public network understands.
- For inbound calls, the process happens in reverse.
Related: Hosted PBX vs. VoIP vs. SIP Trunking
What to look for in a UK SIP trunk provider
When selecting a SIP trunk provider for your global enterprise, prioritize these essential features:
- Global coverage: Access local numbers and place calls in all your markets without multiple providers.
- Regulatory compliance: Meet UK and international telecommunications regulations, such as Office of Communications (Ofcom) requirements and General Data Protection Regulation (GDPR), to avoid service disruptions.
- Call quality: Get crystal-clear audio with minimal latency through optimized routing and Quality of Service (QoS) features.
- Reliability: Ensure uptime guarantees of at least 99.99% with geographic redundancy.
- Security: Protect voice traffic with Transport Layer Security/Secure Real-time Transport Protocol (TLS/SRTP) encryption and fraud prevention tools.
- Scalability: Add or remove channels instantly as business needs change.
- Integration capabilities: Connect seamlessly with your existing communication platforms.
- Analytics and reporting: Track call quality, usage patterns, and costs through intuitive dashboards.
Related: Are ‘Free SIP Trunks’ Really Free?
Best UK SIP trunk providers: How they compare
Here’s a quick comparison of top SIP trunk providers for the UK to help you determine the best fit for your business needs:
| AVOXI | Global voice connectivity, enterprise security (SOC 2, GDPR, HIPAA), easy number porting, advanced analytics | 150+ countries | Free demo |
| BT Cloud | Extensive UK network coverage, legacy PBX compatibility | 70+ countries | No |
| 8x8 | Unified communications integration, TLS/SRTP encryption | 110+ countries | No |
| Vonage | Elastic SIP trunking, UCaaS integration | 90+ countries | 30 days |
| RingCentral | Multiple PBX compatibility, Quality of Service monitoring | 45+ countries | 14 days |
1. AVOXI
AVOXI provides enterprise-grade SIP trunking with coverage in over 150 countries, making it ideal for global businesses transitioning from PSTN/ISDN before the UK switch-off.
Key features:
- Global voice connectivity: Connect directly to local numbers in 150+ countries with premium voice routing.
- Enterprise security: Protect your communications with Systems and Organization Controls 2 (SOC 2), GDPR, and Health Insurance Portability and Accountability Act (HIPAA) compliant solutions.
- Seamless number porting: Retain your existing phone numbers through managed porting services.
- Advanced analytics: Monitor call quality in real time and access comprehensive usage reports.
Pros:
- Global coverage provides more flexibility than most UK-only providers.
- Robust analytics offer deeper insights compared to basic SIP trunking services.
Cons:
- Initial setup requires some technical knowledge.
- Advanced customizations depend heavily on users' familiarity with IP-based phone systems.
2. BT Cloud
As the UK's largest telecommunications provider, BT offers reliable domestic SIP trunking, a solid network infrastructure, and legacy system support.
Key features:
- UK coverage: Provides strong reliability with an extensive domestic network in Great Britain.
- Legacy compatibility: Supports a wide range of existing PBX systems for seamless integration.
- Regulatory expertise: Offers in-depth knowledge of UK telecommunications regulations.
- Bandwidth options: Allows selection from various trunk sizes to meet different business needs.
Pros:
- BT provides domestic coverage with high reliability inside the UK.
- The UK-based support team understands local business needs.
Cons:
- International coverage is more limited than that of global specialists.
- Customization options are fewer compared to globally focused competitors.
3. 8x8
8x8 delivers cloud-based communications services, including SIP trunking, with strong unified communications integration options.
Key features:
- Unified communications integration: Provides connectivity across various communication tools via 8x8's Unified Communications as a Service (UCaaS) platform.
- Local number portability: Enables businesses to retain their existing UK phone numbers when switching services.
- Security focus: Features TLS/SRTP encryption along with fraud prevention tools.
- Simplified management: Includes a user-friendly administration portal for streamlined operations.
Pros:
- Strong security features protect your communications data.
- Cloud-based architecture provides excellent scalability.
Cons:
- The best value comes from using the full 8x8 platform.
- Customer support response times vary by service tier.
4. Vonage
Vonage provides flexible SIP trunking solutions with programmable voice capabilities for businesses that need communications API access.
Key features:
- API access: Provides programmable voice features for custom communications applications.
- Elastic SIP trunking: Adjusts capacity dynamically based on actual usage.
- UCaaS integration: Integrates with the Vonage Business Communications platform.
- Call routing flexibility: Enables customized call handling and routing configurations for UK businesses.
Pros:
- The elastic trunk model helps optimize costs by matching capacity to actual needs.
- Developer-friendly APIs allow for customized communication workflows.
Cons:
- Technical support focuses more on developers than traditional telecom management.
- Complex implementation may require developer resources.
5. RingCentral
RingCentral offers SIP trunking as part of its unified communications ecosystem, which includes strong mobile integration features.
Key features:
- Mobile integration: Provides smooth connectivity for calls transitioning between desk and mobile devices.
- Multiple PBX compatibility: Supports a wide range of IP PBX systems, ensuring easy integration.
- Quality of Service monitoring: Actively measures and delivers high-quality call performance by checking QoS issues like jitter and packet loss.
- UK number porting: Allows businesses to retain their existing UK phone numbers.
Pros:
- The mobile-first approach works well for teams with remote or hybrid employees.
- The admin portal offers simple user management through a centralized system.
Cons:
- RingCentral reserves the best features for full platform users.
- Implementation complexity increases with larger deployments.
5 Benefits of SIP Trunking
SIP trunking is growing rapidly, projected to expand at a compound annual growth rate (CAGR) of 14.13% from 2024 to 2032. In North America, about 65% of companies have already adopted it. The trend is also gaining momentum in the UK and Europe, particularly as the PSTN switch-off pushes businesses toward more modern communications solutions.
Here are the key benefits of using SIP technology:
- Cost savings: SIP trunking cuts communication expenses by up to 60% by eliminating physical lines, hardware, and long-distance charges.
- Scalability: You can add or remove lines instantly based on business needs without installing physical infrastructure.
- Geographic flexibility: Your business can maintain a local presence with local numbers in multiple regions without physical offices.
- Integration with unified communications: SIP trunks connect smoothly with modern collaboration tools, video conferencing, and messaging platforms.
- Enhanced features: You can access advanced and essential capabilities like call recording, voicemail, call forwarding, analytics, and Interactive Voice Response (IVR) systems—no extra hardware needed.
Related: Top Cloud Business VoIP Services Compared (2025)
UK SIP trunking and the 2025 PSTN switch-off
The UK is updating its old analog network to digital due to its aging infrastructure and the challenges of maintaining it. Parts are hard to find since production stopped, and in 2023, service issues increased by 20% over the previous year, causing significant downtime.
Switching to digital phone lines means clearer calls and new features, such as anonymous caller rejection and three-way calling. Digital lines are also more durable during severe weather, thanks to waterproof cables.
Initially set for December 2025, the complete shutdown of the old PSTN and ISDN has been postponed to January 2027. Key milestones in this transition include:
- September 2023: BT stopped selling new PSTN and ISDN lines.
- April 2025: The major migration push begins, accelerating customer transitions.
- January 2027: The start of the scheduled deactivation of PSTN and ISDN services.
This change impacts businesses that rely on ISDN lines for voice and data. To avoid service interruptions, organizations must act swiftly to upgrade their systems.
What does the UK PSTN switch-off mean for your business?
The switch-off means your business must transition to IP-based communications before the deadline to maintain phone service. SIP trunking provides the most direct replacement for existing ISDN lines, allowing you to:
- Keep your existing phone numbers
- Maintain similar call routing and management capabilities
- Continue using compatible PBX systems with SIP gateways
- Avoid service disruption during the national transition
The time to plan your migration is now. Many providers are already reporting busy migration schedules. Waiting until the deadline is closer increases the risk of service disruption and rushed implementation.
Why choose AVOXI for UK SIP trunking
Choosing AVOXI for SIP trunking in the United Kingdom offers several advantages:
- Global reach with local expertise: AVOXI offers seamless SIP trunking coverage in the UK and 150+ countries, ensuring reliable connections and local number availability.
- Enterprise-grade security: Your communications stay protected with SOC 2, GDPR, HIPAA-compliant infrastructure, and Payment Credit Industry (PCI) compliance.
- Premium voice quality: AVOXI's optimized global network ensures crystal-clear calls with minimal latency—even for international connections.
- Transparent pricing: There are no hidden fees or surprise charges with clear, predictable pricing models tailored to your business needs.
- Expert migration support: Dedicated implementation teams help you transition smoothly from PSTN/ISDN before the deadline.
- Scalable connectivity: AVOXI supports unlimited channels, allowing you to scale as your business grows.
- Built-in continuity: With global data centers and failover routing, AVOXI keeps your business online—even during outages.
How to set up SIP trunking with AVOXI
Setting up SIP trunking with AVOXI is straightforward. Follow these steps:
- Schedule a consultation: Connect with AVOXI's solutions team to discuss your specific requirements and global coverage needs.
- Design your solution: Receive a customized implementation plan based on your existing systems, call volumes, and international requirements.
- Configure your PBX: Follow AVOXI's detailed guides to connect your IP PBX to the SIP trunk service with proper security settings.
- Test your connection: Verify call quality, routing, and features before transitioning production traffic.
- Port your numbers: Transfer your existing phone numbers to AVOXI with dedicated porting specialists managing the process.
- Go live and optimize: Transition to new SIP trunks with zero downtime, then use AVOXI's analytics to monitor and adjust call quality.
Future-proof your business by switching to a reliable UK SIP trunk provider now
The 2025 PSTN switch-off creates both a challenge and an opportunity for UK businesses. By transitioning to SIP trunking now, you can avoid the last-minute rush while enjoying more flexible, cost-effective communications.
For global enterprises, selecting the right SIP trunk provider means finding a partner with comprehensive international capabilities, enterprise-grade security, and the technical expertise to ensure a smooth migration.
AVOXI delivers all these requirements with coverage in 150+ countries, premium voice routing, and dedicated implementation support—making it an ideal choice for enterprises seeking a future-proof communications solution.
Secure your business communications with AVOXI's UK SIP trunking. Sign up today for scalable, secure, and high-quality global voice solutions.
