Top 10 SIP Trunking Providers in the UK

SIP trunking connects your existing office PBX to the public phone network over the internet, replacing costly ISDN lines. With BT’s PSTN switch-off in January 2027, every UK business still on traditional phone lines needs to migrate – making the choice of SIP trunk provider one of the most consequential telecom decisions of the decade.

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We compared 10 UK SIP trunk providers on pricing transparency, call quality, failover resilience, Microsoft Teams compatibility, and PSTN migration support. Whether you’re running a 3CX server, an Avaya IP Office, or a Cisco CUCM, this guide covers the providers that matter.

Best UK SIP Trunk Providers at a Glance

The best SIP trunk providers in the UK for 2026 are sipgate (most transparent pricing from £4.95/month), Gradwell (best for 3CX at £3.99/channel), and Gamma (largest independent, via resellers).

1. sipgate Trunking – Most Transparent Pricing

sipgate is the only major UK SIP provider to publish full pricing online – from £4.95/month with no setup fees and monthly rolling contracts.

sipgate is the UK arm of a German VoIP provider and one of the few SIP trunk companies to publish transparent, publicly listed pricing. You can sign up, configure your trunk, and start making calls without ever speaking to a sales team.

All plans include 100 concurrent outbound calls, no setup fees, and inbound calls at no charge. sipgate supports G.711, G.722, and G.729 codecs and is compatible with Asterisk, 3CX, Mitel, Cisco, and most SIP-compatible PBX systems. A free 30-day developer trial is available with 3 numbers and 60 minutes included.

2. Gradwell – Best for 3CX Users

Gradwell is a Bath-based provider with 3CX Titanium Partner status, ISO 27001 certification, and SIP trunks from £3.99 per channel per month.

Founded in 1998, Gradwell is one of the most established independent SIP providers in the UK. Their 3CX Titanium Partner status means pre-built templates and certified compatibility – if you’re running 3CX, Gradwell should be on your shortlist.

Gradwell hosts its infrastructure across AWS with multiple carrier connections for resilience. They offer Microsoft Teams Direct Routing, number porting with a typical 2-week turnaround, and geographic, non-geographic, and international number provisioning. Trustpilot ratings consistently sit at 4/5.

3. Gamma – Largest UK Independent Provider

Gamma is the UK’s largest independent telecoms provider, supplying SIP trunks exclusively through a reseller channel to businesses of all sizes.

If you’re buying SIP trunks through a managed service provider, there’s a good chance Gamma is the underlying carrier. They’re the dominant force in UK SIP trunking, but they don’t sell direct – you’ll need to go through a reseller like Connexis, FourNet, Focus Group, or Onecom.

Gamma’s SIP Trunk Call Manager is a standout feature – a cloud portal that adds IVR, time-of-day routing, and call reporting on top of your existing PBX. Their failover options are also among the most sophisticated in the market, with Resilient Mesh connecting multiple sites for automatic rerouting during outages.

4. BT Business – The Incumbent

BT Business offers SIP trunks from approximately £13 per trunk per month with unlimited UK calls, backed by end-to-end BT network infrastructure.

As the UK’s largest telecoms provider and the company driving the PSTN switch-off, BT carries unmatched network reach. Their Cloud Voice SIP product runs entirely on BT’s own infrastructure – not riding third-party networks – which gives them quality control advantages that smaller providers cannot match.

BT stopped selling ISDN/PSTN in all exchange areas from September 2023 and is actively migrating its own ISDN customer base to Cloud Voice. If you’re already a BT customer, the migration path is smoother than switching providers entirely. However, BT’s premium pricing means you’ll pay 2–3x more per channel than independent alternatives like sipgate or Gradwell.

5. Spitfire – Best for End-to-End Quality

Spitfire owns its own UK network and offers SIP trunks from £4 per channel, with priority QoS when bundled with Spitfire connectivity.

Unlike most SIP providers that ride on BT or Gamma’s network, Spitfire operates its own infrastructure. This means when you buy SIP trunks and connectivity together, Spitfire controls quality from your office to the public network – a genuine advantage for businesses where call quality is non-negotiable.

Spitfire’s SIP Gateway product is specifically designed for businesses upgrading legacy ISDN PBX systems to SIP without replacing hardware – a direct response to PSTN switch-off demand. They also support Microsoft Teams Direct Routing and are compatible with all major PBX vendors.

6. Andrews & Arnold (AAISP) – Best for Technical Teams

AAISP is a technically rigorous UK ISP offering SIP trunks with no concurrent call limits, IPv6 by default, and monthly rolling contracts from £1.56 per number.

If your team runs its own Asterisk or FreePBX server, AAISP speaks your language. They’re transparently technical, unusually honest (Trustpilot 4.7+), and impose no concurrent call limits – your capacity is constrained only by your internet bandwidth. This makes them exceptionally good value for businesses with high concurrent call volumes.

AAISP supports DNS SRV records, flexible CLI presentation, and authenticates via international-format usernames. Number porting is available, though at a higher one-off cost than provisioning new numbers. This is a provider for businesses that know what a codec is and want maximum control.

7. 8×8 – Best for UCaaS + SIP Hybrid

8×8 combines SIP trunking with a full UCaaS platform, including Microsoft Teams Direct Routing and unlimited calling to 47 countries from approximately £12 per user per month.

If your business needs both SIP trunking for existing PBX hardware and a unified communications platform for remote workers, 8×8 bridges the gap. Their Voice for Microsoft Teams product is one of the most mature Teams Direct Routing integrations available.

8. Zen Internet – Best Connectivity Bundle

Zen Internet bundles SIP trunking with business broadband and leased lines, offering QoS advantages for businesses buying voice and data together.

Rochdale-based Zen Internet has been a trusted UK ISP since 1995, consistently winning Which? Recommended Provider status. Their SIP trunk offering is strongest when bundled with Zen connectivity – giving priority to voice traffic across the same infrastructure, similar to Spitfire’s approach.

9. NFON (Nconnect Voice) – Best for Legacy PBX Migration

NFON’s Nconnect Voice product connects existing IP or legacy PBX systems to a carrier-grade SIP network, designed specifically for PSTN switch-off migration.

Munich-headquartered NFON sells through UK channel partners and positions Nconnect Voice as an on-ramp from legacy PBX to their Cloudya cloud platform. If you want to keep your existing PBX but need to replace ISDN, this is worth considering.

10. Vonage (Ericsson) – Best for Global and Developer Use

Vonage offers elastic SIP trunking with global number coverage across 140+ countries, scaling channels instantly via API – ideal for businesses with international operations or custom communications workflows.

Acquired by Ericsson for $6.2 billion in 2022, Vonage’s SIP product is primarily developer-focused through their Communications APIs platform. UK business plans start from approximately £15 per line per month, but the real differentiator is elastic SIP trunking – scaling channels on demand via API without capacity planning.

Vonage holds two UK legal entities (Vonage Limited and Vonage Business Limited) with a European HQ in Shoreditch, London. UK sales: 0800 008 6800.

SIP Trunk Costs Compared

UK SIP trunk costs range from £3.99 to £13+ per channel per month, with typical call rates of 1–2p/min to UK landlines and 8–12p/min to UK mobiles.

Cost comparison with ISDN: Switching from ISDN to SIP trunking typically saves 40–60% on monthly voice spend. A business with 30 ISDN channels paying ~£300/month could reduce to £120–180/month on SIP – plus lower call rates and no per-line rental charges.

SIP Trunking vs Hosted VoIP: When to Choose Each

Choose SIP trunking if you have an existing PBX worth keeping. Choose hosted VoIP if you’re starting fresh or want a fully managed cloud solution.

The key question: Do you have a working Avaya, Cisco, Mitel, or 3CX PBX that cost £10,000–50,000 to install? SIP trunking replaces just the lines connecting it to the public network – your PBX stays, only the connectivity changes. If you don’t have a PBX or want video conferencing, team messaging, and mobile apps built in, a hosted VoIP provider is the better choice.

The PSTN Switch-Off: What It Means for SIP Trunking

BT will switch off all PSTN and ISDN lines by January 2027, forcing approximately 2.4 million UK businesses to migrate to SIP trunking or hosted VoIP.

Key dates:

• September 2023: BT stopped all new PSTN/ISDN sales in all exchange areas
• January 2027: All analogue and ISDN lines cease to function

What stops working: Traditional analogue phone lines, ISDN2 and ISDN30 circuits, ADSL broadband (uses copper voice line), phone-line-based alarm systems, lift phones, and legacy EPOS terminals on PSTN.

What to do now: Start number porting at least 6 months before the deadline (by July 2026) to avoid bottlenecks. BT recommends completing migration by December 2026 as a safety buffer. SIP trunking is the primary like-for-like replacement for ISDN30 circuits – same per-channel billing, compatible with existing PBX hardware.

Technical Requirements for SIP Trunking

A SIP trunk requires an IP-compatible PBX, business-grade internet with at least 90 kbps per concurrent call, and optionally a Session Border Controller for enterprise deployments.

Bandwidth per Concurrent Call

Internet Connection Sizing

Network Quality Thresholds

Do you need a Session Border Controller (SBC)? For an SME running 3CX or FreePBX on a single site, no – modern IP-PBX systems handle NAT traversal natively. For enterprise multi-site deployments with Avaya or Cisco, or for Microsoft Teams Direct Routing, a certified SBC (AudioCodes, Ribbon, or Oracle) is mandatory.

How to Choose a SIP Trunk Provider

Prioritise providers with verified UK pricing, PSTN migration support, your PBX on their compatibility list, and failover capabilities that match your uptime requirements.

For businesses running modern phone systems, check whether your provider already offers SIP connectivity – you may not need a separate trunk provider at all. RingCentral, Dialpad, and bOnline include their own connectivity as part of a hosted VoIP package.

If you’re unsure whether SIP trunking or hosted VoIP is right for your business, our VoIP comparison guide covers the alternatives.

Our Verdict

For most UK SMEs, Gradwell (3CX users) or sipgate (self-service) offer the best combination of transparent pricing, proven reliability, and PSTN migration support.

Best overall value: sipgate – transparent pricing from £4.95/month, no contracts, free inbound calls. The only provider where you can compare plans without contacting sales.

Best for 3CX: Gradwell – Titanium Partner status, £3.99/channel, ISO 27001 certified. If your PBX is 3CX, this is the natural fit.

Best for enterprise: Gamma – the UK’s largest independent, with Resilient Mesh failover and Teams Operator Connect. You’ll need a reseller, but the underlying infrastructure is the best in the UK market.

Best for BT customers: BT Business – if you’re already on BT ISDN, the migration to BT Cloud Voice SIP is the path of least resistance. You’ll pay more, but the transition is simpler.

Best for technical teams: AAISP – no call limits, monthly rolling, IPv6 native. Unbeatable control for businesses running their own PBX infrastructure.

The PSTN switch-off deadline is January 2027. If your business still relies on ISDN or analogue lines, start your SIP migration now – number porting alone can take 2–4 weeks, and demand will spike as the deadline approaches.

Emma Clarke

Technology & Payments Specialist

Emma covers the full range of business technology, including EPOS systems, merchant accounts, telecoms, and web tools. Her experience as a retail systems consultant helps businesses choose the right digital solutions to improve efficiency and sales.

Reviewed by

Sarah Mitchell

B2B Commerce & Finance Reviewer