Quick Answer
SIP trunking is a method of delivering phone calls over the internet using the Session Initiation Protocol (SIP). Instead of traditional copper phone lines, a SIP trunk connects your existing PBX phone system to the public telephone network via your broadband connection.
It lets UK businesses keep their current desk phones and PBX hardware while cutting call costs by 30–50%, adding flexible capacity and future-proofing ahead of the 2027 PSTN switch-off.
Updated March 2026
If you have a PBX phone system in your office and you are still paying for ISDN lines, SIP trunking is almost certainly the most cost-effective upgrade you can make right now.
With the PSTN switch-off arriving in January 2027, every business on legacy phone lines will need to move to an IP-based alternative. SIP trunking is one of the two main options — the other being hosted VoIP.
This guide explains what SIP trunking is, how it works, what it costs in 2026, and how to decide whether it is the right choice for your business.
What Is SIP Trunking?
SIP stands for Session Initiation Protocol. It is the standard technology used to set up, manage and tear down voice calls over the internet.
A SIP trunk is a virtual phone line that replaces the physical copper or fibre lines (ISDN) that traditionally connected your office PBX to the telephone network.
Think of it this way: your PBX is the brain of your phone system. It handles extensions, call routing, voicemail and all the clever stuff. Traditionally, it connected to the outside world through ISDN lines — physical cables running from the telephone exchange to your building.
A SIP trunk does exactly the same job, but the connection runs over your broadband instead of dedicated copper pairs.
The result is that you keep your existing PBX, your desk phones, your extension numbers and your way of working. The only thing that changes is how calls get in and out of the building — and you pay significantly less for the privilege.
For a deeper technical explanation, see our complete SIP trunking technical guide.
How SIP Trunking Works
The technical process behind a SIP trunk call is straightforward:
- Call initiation: When someone in your office picks up the phone and dials an external number, the PBX creates a SIP INVITE message containing the destination number, codec preferences and caller ID information.
- Trunk connection: The SIP INVITE travels over your broadband connection to your SIP trunk provider’s servers (known as a Session Border Controller or SBC).
- PSTN gateway: The provider’s SBC routes the call to the public telephone network (PSTN), connecting it to the destination — whether that is a landline, mobile or international number.
- Media stream: Once the call is established, the actual voice data (RTP packets) flows directly between your PBX and the provider’s network. Each concurrent call uses approximately 80–100 Kbps of bandwidth.
- Call termination: When either party hangs up, a SIP BYE message ends the session and releases the channel for the next call.
Inbound calls work in reverse: the provider receives the call from the PSTN, converts it to SIP and delivers it to your PBX over the internet.
Key point: Each SIP trunk channel supports one concurrent call. If you need 10 people to be on external calls simultaneously, you need 10 channels. Unlike ISDN (which came in fixed blocks of 2 or 30), SIP trunk channels can be added one at a time, so you only pay for exactly what you need.
If your PBX sits behind a firewall, you will need to configure it to allow SIP traffic. Our SIP trunk firewall configuration guide covers the ports and settings you need.
SIP Trunking Costs UK 2026
SIP trunking is significantly cheaper than ISDN. Here is what UK businesses can expect to pay in 2026:
| Cost Element | Typical Range | Notes |
|---|---|---|
| Per channel (monthly) | £2.50 – £6.00 | Price drops with volume; 10+ channels often £3 or less |
| Setup fee | £0 – £50 | Many providers waive setup fees; some charge a one-off config fee |
| UK landline calls | 0.5p – 1.5p/min | Many plans include bundled UK minutes |
| UK mobile calls | 2p – 5p/min | Some plans bundle mobile minutes too |
| Number porting | £0 – £10 per number | Most providers port numbers free of charge |
| DDI numbers | £0.50 – £1.50/month each | Direct dial-in numbers for individual extensions |
Example: A business with 8 SIP trunk channels, 10 DDI numbers and moderate call usage would typically pay £35–£60 per month. The equivalent ISDN setup (ISDN30 with 8 channels) would cost £250–£350 per month. That is a saving of around £200 per month — or £2,400 per year.
SIP Trunking vs ISDN
ISDN has been the standard business phone line technology in the UK for over 30 years. Here is how it compares to SIP trunking:
| Feature | SIP Trunking | ISDN |
|---|---|---|
| Cost per channel | £3–£6/month | £15–£30/month |
| Channel flexibility | Add/remove one at a time | Fixed blocks (2 or 30) |
| Call rates (UK landline) | 0.5p–1.5p/min | 2p–8p/min |
| Setup time | 1–3 days | 2–6 weeks (engineer visit) |
| Scalability | Instant — add channels online | Slow — requires new lines |
| Disaster recovery | Reroute calls instantly to any location | Tied to physical location |
| Future availability | Long-term standard | Switched off January 2027 |
| Number porting | Supported | N/A |
The bottom line: ISDN is being permanently switched off in January 2027. Even if cost were not a factor, every business on ISDN must migrate. SIP trunking is the natural replacement if you want to keep your existing PBX.
SIP Trunking vs Hosted VoIP
The other major alternative to ISDN is hosted VoIP (also called cloud PBX). Here is how the two compare:
| Feature | SIP Trunking | Hosted VoIP |
|---|---|---|
| PBX location | On-premise (your hardware) | Cloud (provider’s data centre) |
| Best for | Businesses with existing PBX hardware | Businesses starting fresh or replacing old systems |
| Upfront cost | Low (reuse existing PBX) | Low–medium (new IP phones may be needed) |
| Monthly cost | £3–£6 per channel | £6–£15 per user |
| Maintenance | You maintain the PBX | Provider handles everything |
| Features | Depends on your PBX capabilities | Full feature set included (apps, integrations, UC) |
| Scalability | Limited by PBX capacity | Unlimited — add users instantly |
| Control | Full control over PBX configuration | Provider-managed with admin portal |
Which should you choose? If you have a modern, well-functioning PBX that you have already invested in, SIP trunking lets you protect that investment while slashing line rental and call costs.
If your PBX is old, end-of-life or you simply want someone else to manage the phone system, hosted VoIP is the cleaner option. Many businesses with ageing PBX hardware use SIP trunking as a bridge while planning a full migration to hosted VoIP.
Benefits of SIP Trunking for Business
Here are the practical advantages that make SIP trunking attractive to UK businesses:
- Cost reduction of 30–50%: Lower line rental, cheaper call rates and no engineer visits for changes. Most businesses see a return on investment within the first month.
- Flexible capacity: Add or remove channels instantly to match seasonal demand. No more paying for 30 ISDN channels when you only use 12.
- Number portability: Keep all your existing phone numbers. Geographic numbers, non-geographic numbers and DDIs all transfer seamlessly.
- Business continuity: If your office loses power or internet, calls can be automatically rerouted to mobiles, another office or a voicemail system within seconds.
- Multi-site consolidation: Connect multiple offices through a single SIP trunk provider with centralised billing and management, regardless of where each office is located.
- Future-proof: SIP is the standard protocol for voice communications going forward. Moving to SIP trunking now means you are ready for the PSTN switch-off with no last-minute scramble.
- Protect existing investment: Your PBX, desk phones, cabling and configuration all stay exactly as they are. Only the external connection method changes.
- Better call quality: Modern SIP trunks use HD voice codecs (G.722) that deliver noticeably clearer audio than traditional ISDN calls.
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The PSTN Switch-Off and Why SIP Trunking Matters Now
Openreach is permanently switching off the Public Switched Telephone Network (PSTN) and all ISDN services by January 2027. This affects every business in the UK that uses traditional phone lines.
Openreach stopped selling new ISDN lines in September 2023, and the full shutdown is now less than a year away.
For businesses with a PBX, sip trunking is the most straightforward migration path. You do not need to replace your phone system, retrain staff or change your phone numbers. The transition can be completed in days rather than weeks, and you start s
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Frequently Asked Questions
What is SIP trunking and how does it work for UK business?
SIP trunking delivers voice calls over your existing internet connection rather than over traditional phone lines. SIP trunks connect your on-premise phone system (PBX) to the public phone network via a SIP provider, usually using G.711 or G.729 codecs. For UK businesses still running on-prem PBX hardware, SIP trunking is the standard way to keep that hardware while removing the cost of ISDN30 or analogue lines. Most modern PBX systems (Avaya, Mitel, Cisco, NEC, 3CX) support SIP natively.
How much do SIP trunks cost in the UK in 2026?
UK SIP trunk channel pricing in 2026 typically runs £4–£8 per channel per month with calls bundled (e.g. unlimited UK landline + 2,000 mobile minutes for an additional £4–£6/channel/month). For comparison, ISDN30 costs around £55–£75 per channel per month plus separate call charges. A 10-channel SIP trunk replacement typically saves £6,000–£10,000/year versus the equivalent ISDN30, before factoring in the call-cost savings.
Do I need SIP trunking if I have a hosted VoIP system?
No — hosted VoIP (cloud PBX) replaces both the PBX and the SIP trunks with a single cloud service. SIP trunking is for businesses that still run on-prem PBX hardware they want to keep — typically because of large existing investment, complex call routing, or specific compliance requirements. For most UK SMEs in 2026, hosted VoIP is the cleaner and cheaper choice; SIP trunking is the migration path for businesses with sunk cost in PBX hardware.
What broadband do I need for SIP trunking?
Plan for around 100 Kbps per concurrent call (G.711 codec) plus a 30% headroom for jitter and network overhead. A 10-channel SIP trunk needs ~1.3 Mbps reserved bandwidth in each direction, which any modern fibre connection handles easily. The critical requirement is low jitter (<30 ms) and packet loss below 1% — usually requiring QoS configuration on your router/firewall to prioritise SIP traffic.
Is SIP trunking secure for business use?
Yes when configured correctly. Best practice: use TLS for SIP signalling, SRTP for media, restrict the SIP trunk to specific IP addresses on your firewall, use strong outbound caller-ID authentication (STIR/SHAKEN equivalent), and monitor for unusual call patterns indicating toll fraud. Toll fraud remains the biggest SIP risk — typically attackers exploit weak credentials to make international calls overnight. SIP providers should let you cap international destinations and set spend limits.
Can I keep my existing phone numbers when moving to SIP?
Yes — SIP providers port-in existing UK numbers (geographic 01/02 and non-geographic 0800/03) using standard porting agreements. Porting takes 5–10 working days for single numbers, 10–25 working days for full geographic ranges. Mobile numbers cannot be ported into SIP trunks but can be diverted via a separate IVR setup. Number porting is typically free.
What is the difference between SIP trunking and PSTN/ISDN?
PSTN (analogue) and ISDN (digital) are physical phone lines provided by Openreach. SIP trunking is a virtual phone “line” delivered over your internet connection. PSTN is being switched off in the UK in 2027 — all analogue and ISDN services will stop working. Businesses still on ISDN must migrate to SIP trunking or hosted VoIP before then. SIP trunking is the like-for-like replacement; hosted VoIP is a more comprehensive replacement.
How do I migrate from ISDN to SIP trunking?
Standard four-step migration: (1) audit current PBX, channel count and number range; (2) provision SIP trunks with the same channel count and have your provider configure your PBX (usually a one-line config change); (3) port numbers from BT/Openreach to the SIP provider on a scheduled date; (4) decommission ISDN once SIP is stable. The cutover itself is typically a 2-hour window during which calls continue normally — well-rehearsed migrations have zero downtime.
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