What Is A SIP Trunk Provider And Do I Need One?

By Cedar Telecom on Oct 22, 2026

If you've been looking at modernising your business phone system, or you've been told that ISDN lines are being switched off and you need to do something about it, you've probably come across the term SIP trunking. It sounds technical, but the concept is straightforward. Here, we explain what SIP trunk providers actually are, how SIP trunking works, and how to tell whether your business needs it.

What Is A SIP Trunk Provider?

To understand what a SIP trunk provider does, it helps to understand what SIP trunking replaces. Traditional business phone systems have historically connected to the public phone network through physical lines. Those lines are being switched off across the UK as part of the Big Switch Off, with BT Openreach planning to complete the migration away from legacy public switched telephone network (PSTN) services by January 2027.

SIP trunking is the modern replacement. SIP stands for Session Initiation Protocol, which is the technology that handles the signalling and control of voice calls over an internet connection. Instead of a physical line connecting your phone system to the outside world, a SIP trunk is a virtual connection that carries calls over the internet. In plain terms: your phone calls travel over your broadband or dedicated internet connection rather than a separate phone line.

A SIP trunk provider is the company that supplies and manages those virtual call channels, connecting your existing phone system to the public telephone network via the internet. They handle the underlying infrastructure so that calls can be made and received in the same way they always have been, just without the physical lines.

How Does It Actually Work?

Most businesses that use SIP trunking have what's called a PBX (Private Branch Exchange) on site. This is the hardware that manages internal phone extensions and routes calls to and from the outside world. Historically, the PBX connected to the wider network through ISDN lines. With SIP trunking, calls connect through a SIP trunk supplied by your SIP provider, which routes them over your internet connection.

The number of SIP trunks you need depends on how many simultaneous calls your business needs to handle. If you typically have up to ten people on external calls at once, you'd provision ten SIP trunks. You can scale up or down relatively easily, which is one of the practical advantages over fixed ISDN lines where each line had a set capacity you've already paid for.

Call quality over SIP depends on the quality of your internet connection. A reliable, well-provisioned connection with low latency will deliver good voice quality. A congested or variable connection can result in call quality problems, which is why businesses running SIP trunking for a busy phone system often consider a dedicated leased line or, at minimum, a business-grade broadband connection with quality of service settings configured to prioritise voice traffic.

Do I Need A SIP Trunk Provider?

Whether your business specifically needs a SIP trunk provider depends on how your current phone system is set up.

You're likely to need SIP trunking if:

1. You have an on-site PBX that currently connects to the outside world through ISDN lines, because those lines are being switched off, and you'll need an alternative.
2. You want to keep your existing PBX investment but move away from physical phone lines.
3. You're looking for more flexibility in how many call channels you have, without being locked into a fixed number of physical lines.
4. You want to reduce line rental costs compared to traditional ISDN.

Many businesses are choosing to move to a fully hosted or cloud-based phone system, where calls are managed entirely in the cloud, and there's no on-site hardware involved. In that model, the hosted system includes the call routing to the public network as part of the service, so you don't need a separate SIP trunk provider. The right choice depends on whether you want to retain your existing PBX or move to a fully cloud-based solution.

The Big Switch Off And Why It Matters

The upcoming end of ISDN services isn't a distant concern. BT Openreach is actively progressing the withdrawal of PSTN and ISDN services, with the full switch-off planned for January 2027. Businesses that haven't yet moved away from ISDN-connected phone systems will need to act before that deadline.

For businesses with on-site PBX systems, SIP trunking is typically the most practical migration route because it allows the existing phone system to continue working without the need for immediate hardware replacement.

For businesses that take the opportunity to review their telephony setup more broadly, the switch-off is also a natural moment to consider whether a fully hosted or cloud-based phone system might be a better long-term fit.

If you're not sure which direction makes sense for your business, or you want a clear picture of what's involved in moving away from ISDN, get in touch with a member of our team today.

Image Source: Envato