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

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 saving money immediately.

Businesses that leave the migration too late risk being moved to a basic digital voice service by their provider — which may not support PBX connectivity, DDI numbers or the call features they rely on. Acting now gives you time to choose the right provider, test the service and resolve any issues before the deadline.

Read our full guide: What is the PSTN switch-off and how does it affect your business?

How to Set Up SIP Trunking

Setting up SIP trunking is simpler than most businesses expect. Here is the step-by-step process:

Step 1: Check Your PBX Compatibility

Most modern PBX systems support SIP trunking natively. If your PBX is less than 10–12 years old, it almost certainly has a SIP interface. Older systems may need a SIP gateway (a small device costing £100–£300) to convert between legacy protocols and SIP. Your provider can confirm compatibility.

Step 2: Assess Your Broadband

Each concurrent SIP call uses approximately 100 Kbps of bandwidth. For 10 simultaneous calls, you need about 1 Mbps of dedicated bandwidth. Most business broadband connections (even basic fibre at 40 Mbps) handle this comfortably. If you have a very high call volume, consider a connection with a dedicated bandwidth allocation or QoS (Quality of Service) settings.

Step 3: Choose a SIP Trunk Provider

Compare providers on price, reliability, support quality and contract terms. Key questions to ask: What is the uptime SLA? Is UK-based support included? Do they support number porting? What codecs do they offer? Are there minimum contract terms?

Step 4: Configure Your Firewall

SIP trunking requires specific ports to be open on your firewall. Typically, you need to allow SIP signalling (UDP/TCP port 5060 or 5061 for TLS) and RTP media (UDP ports 10000–20000, though the exact range varies by provider). See our firewall configuration guide for detailed instructions.

Step 5: Configure Your PBX

Your provider will supply SIP trunk credentials (server address, username, password and authentication details). These are entered into your PBX’s trunk configuration. Most providers also supply step-by-step guides for popular PBX platforms including Avaya, Mitel, Panasonic, 3CX and Asterisk.

Step 6: Port Your Numbers

Submit a number porting request through your new provider. They handle the process with your current carrier. Porting typically takes 5–10 working days for geographic numbers. During the port, your numbers continue working on the old service with no interruption.

Step 7: Test and Go Live

Before porting, test inbound and outbound calls using temporary numbers supplied by your provider. Check call quality, caller ID presentation, voicemail and any call routing rules. Once you are satisfied, confirm the port and go live.

UK SIP Trunk Providers Compared

Here is an overview of the main SIP trunk providers serving UK businesses in 2026:

How to choose: For most small and mid-sized businesses, Gamma and Voiceflex offer the best balance of reliability, pricing and support. If budget is the primary concern and you have in-house technical expertise, Magrathea offers the lowest per-channel rates. BT Wholesale suits businesses that want a single-provider relationship and are less price-sensitive.

Frequently Asked Questions

Can I keep my existing phone numbers with SIP trunking?

Yes. Number porting transfers your existing geographic numbers, non-geographic numbers and DDIs to your new SIP trunk provider. The process takes 5–10 working days and is seamless — callers will not notice any change.

How many SIP channels do I need?

You need one channel for each concurrent external call. A common rule of thumb is one channel for every three to four employees, but this varies depending on your call patterns. Most providers let you start with a conservative number and add channels instantly if needed.

Is SIP trunking reliable?

Modern SIP trunking services offer 99.9%+ uptime. Because calls route over the internet, you also gain disaster recovery options that ISDN cannot match — if your office goes offline, calls can be rerouted to mobiles or another site within seconds.

What happens if my broadband goes down?

Most SIP trunk providers offer automatic failover. If your primary internet connection fails, calls are redirected to a backup destination (mobile numbers, another office, voicemail) that you configure in advance. For mission-critical environments, a backup broadband connection provides full redundancy.

Do I need to replace my PBX?

No. If your PBX supports SIP (most systems manufactured after 2012 do), you simply configure a SIP trunk connection. Older PBX systems can use a SIP gateway device to bridge the gap. The whole point of SIP trunking is to keep your existing phone system while changing only the external connection.

Related reading:

• What is SIP trunking? — Technical guide
• SIP trunk firewall configuration
• What is the PSTN switch-off?
• Hosted VoIP for business UK 2026