SIP Trunking UK 2026: Costs, Setup and How It Works

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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:

1. 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.
2. 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).
3. 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.
4. 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.
5. 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:

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:

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:

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.

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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Written by

Charlotte Traviss

Head of Retentions

Charlotte is the Head of Retentions at Connection Technologies, responsible for leading customer retention strategies and ensuring long-term client satisfaction.

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