SIP trunking replaces physical phone lines, such as ISDN, with virtual lines delivered over the internet. It lets you keep your existing business phone system while moving your calls onto a digital connection.

For UK businesses this is timely. The legacy copper phone network is due to be retired on 31 January 2027, and Openreach has said it will not be delayed again. If your PBX still relies on ISDN lines, you need a replacement, and SIP trunking is one of the two main routes, alongside moving to a hosted system.

This guide explains how it works and how to choose a provider sensibly.

What SIP Trunking Actually Is

SIP stands for Session Initiation Protocol, the standard used to set up and end voice calls over IP networks. The UK SIP trunk is the virtual equivalent of a bundle of phone lines. Your PBX connects to your provider over your internet connection, and the provider connects those calls to the wider telephone network.

Your PBX can be a modern IP system on your premises. An older PBX can often work too, usually with a gateway device that converts its signals into SIP. Your team keeps the handsets and features they know.

Channels and Numbers

Two terms come up in every quote:

  • Channels are the number of calls you can make or take at the same time. A ten-channel trunk handles ten simultaneous calls.
  • DDI numbers are direct dial-in numbers that route calls to specific people or departments. You can usually keep existing numbers through number porting.

Getting the channel count right matters. Too few and callers hear busy tones at peak times. Too many and you pay for capacity you don’t use.

SIP Trunking vs a Hosted Phone System

This is the question most buyers face, and the answer depends on what you already own.

SIP trunking suits businesses with a PBX they want to keep, perhaps because it’s recent, heavily customised or integrated with other systems. You only replace the lines.

A hosted system moves the PBX itself into the provider’s cloud. It suits businesses that want to remove on-site equipment and manage everything through a web portal.

Neither is better in every case. Some firms start with SIP trunks to meet the switch-off deadline, then move to hosted telephony when their PBX reaches end of life. A good provider will explain both options rather than push one.

What to Check When Choosing a UK Provider

Network and Call Quality

Voice quality depends on the connection between you and the provider. Ask whether they recommend a dedicated leased line, business fibre or an assured quality path, and how they prioritise voice traffic. A provider that asks about your existing network before quoting is usually a good sign.

Resilience and Failover

If your internet connection fails, so do your trunks, unless you’ve planned for it. Ask about:

  • Automatic diversion of calls to mobile numbers or another site
  • Redundant routes or a second connection
  • Battery backup for your router and gateway
  • How quickly the provider responds to faults

Compatibility With Your PBX

Not every PBX works with every provider. Ask for confirmation that your system is supported, and whether the provider has tested it. If you need a gateway, find out who supplies and configures it.

Number Porting

Check the realistic timeline, what happens to numbers during the switchover and whether you can keep old lines live until the new ones are confirmed. Poor porting plans cause more disruption than the technology itself.

Call Charges and Pricing

Pricing usually includes a per-channel rental, DDI number fees and call charges. Compare costs for UK landlines, mobiles, international destinations, and 03 and 08 numbers. Ask whether bundles are included, and what overage rates apply.

Security

SIP fraud, where criminals exploit weak configuration to make costly calls, is a genuine risk. Ask about call spending limits, alerts for unusual activity, encryption options and whether the provider uses session border controllers to protect traffic.

Emergency Calls

Check how 999 calls are handled, and how your registered address information is kept accurate, especially if you have several sites or remote workers. Ask for this in writing.

Support and Contract Terms

Look at UK-based support hours, response times and who is responsible when something fails. Check minimum terms, notice periods and the cost of changing channel numbers.

A Practical Example

Imagine a 40-person engineering firm in the Midlands with a three-year-old IP PBX and an ISDN30 line. They don’t want to replace the phone system, but the lines need to go.

They choose a SIP trunk with twenty channels, port their existing numbers, add a second connection for failover and route calls to managers’ mobiles if the office goes offline. Staff keep the same handsets and extensions. Two years later, when the PBX needs replacing, they consider a hosted system using the same provider.

Planning Your Migration

  1. Audit your lines. List all numbers, channels and devices. Alarms, lift lines, CCTV and card terminals often use phone lines too.
  2. Check your PBX. Confirm whether it supports SIP natively or needs a gateway.
  3. Review your network. Test bandwidth and stability, and decide whether you need a better connection.
  4. Choose a channel count based on peak call volumes, not total staff numbers.
  5. Agree porting dates and keep legacy lines live until the switch is confirmed.
  6. Test before go-live, including failover, inbound and outbound calls and 999 routing.

Start early. Provider and engineer capacity tends to tighten as the January 2027 deadline approaches. Specialists such as Wavetel Business work with UK organisations on migrations like this, and any provider you consider should be willing to put their plan and pricing in writing.

Frequently Asked Questions

What is SIP trunking in simple terms?

It’s a way of delivering business phone lines over the internet instead of through ISDN or analogue lines. Your existing PBX connects to a provider, which links your calls to the telephone network.

Do I need to replace my PBX?

Not necessarily. Many IP PBXs support SIP directly, and older systems can often use a gateway. Ask your provider and your PBX supplier to confirm compatibility.

How many channels do I need?

It depends on how many simultaneous calls you handle at peak times. Review call logs if you have them, then add a small buffer.

Is SIP trunking cheaper than ISDN?

Often it is, because channels and calls are typically priced more flexibly. Savings depend on your call patterns and the quality of connection you need, so compare complete quotes.

What happens if my internet goes down?

Your trunks are affected unless you have a failover. Ask about automatic call diversion and consider a backup connection.

Conclusion

SIP trunking lets businesses keep their existing phone system while moving to a digital connection. It’s a practical option when your PBX still has life left, and a stepping stone to hosted telephony later on.

The takeaway is to judge providers on resilience, PBX compatibility, porting, security and support, not just price per channel. Audit your lines now, ask detailed questions and plan the move well before January 2027.