What Is SIP Trunking? A Simple Business Guide

If you’ve been told your business needs to switch from ISDN to SIP trunking, you’re not alone. With the UK PSTN switch-off deadline fast approaching in 2027, thousands of businesses are making the move — but many aren’t entirely sure what SIP trunking actually is, how it works, or whether it’s the right choice for them.

This guide cuts through the jargon. Whether you’re a managing director, office manager, or IT lead, we’ll explain SIP trunking in plain English, break down the costs, and help you decide whether it’s the best fit for your organisation.

What Is SIP Trunking?

SIP trunking is a way of making and receiving phone calls over the internet rather than through traditional copper telephone lines. SIP stands for Session Initiation Protocol — it’s the technology standard that sets up, manages, and ends voice (and video) calls over an internet connection.

Think of a SIP trunk as a virtual phone line. Instead of plugging physical cables from BT’s exchange into your office PBX (Private Branch Exchange — your on-site phone system), a SIP trunk delivers your calls over your broadband or leased line connection.

The result? You keep your existing phone system hardware, keep your phone numbers, and typically save a significant amount of money — all while getting a more flexible, future-proof setup.

How Does SIP Trunking Work? A Simple Breakdown

Here’s a simplified view of how SIP trunking connects your business to the outside world:

Let’s walk through what happens when someone in your office makes a call:

1. The call is initiated — A member of staff dials a number on their desk phone, just as they always have.
2. Your PBX routes the call — Your on-site phone system handles the call as usual (extensions, hunt groups, voicemail, etc.) and sends it out via the SIP trunk instead of an ISDN line.
3. The call travels over the internet — Voice data is converted into small packets and sent over your internet connection to your SIP trunking provider.
4. The provider connects the call — Your SIP provider routes the call to the recipient, whether they’re on a mobile, a landline, or another VoIP system.

From the caller’s perspective, absolutely nothing changes. The phone rings, the call connects, and the voice quality is excellent — often better than traditional ISDN.

SIP Trunking vs ISDN: Why Businesses Are Switching

For decades, ISDN (Integrated Services Digital Network) was the standard way UK businesses connected their phone systems to the outside world. But ISDN is now end-of-life technology. Here’s how SIP trunking compares:

The savings alone make a compelling case. A business with 30 ISDN channels paying £20 per channel per month could reduce that to as little as £3 per channel per month with SIP trunking — a saving of over £6,000 per year, before you factor in reduced call charges.

The 2027 PSTN Switch-Off: Why This Matters Now

BT Openreach has confirmed that the Public Switched Telephone Network (PSTN) and all ISDN services will be permanently switched off by January 2027. This isn’t a rumour or a distant possibility — the stop-sell on new ISDN lines has already happened across much of the UK.

This means:

• You cannot order new ISDN lines in most exchange areas.
• Existing ISDN lines will stop working entirely by 2027.
• Every business still using ISDN must migrate to an IP-based alternative — SIP trunking or hosted VoIP.

Waiting until the last minute is risky. As the deadline approaches, demand for migration services will surge, and businesses that plan ahead will secure better pricing, smoother transitions, and zero downtime. At Connection Technologies, we’re already helping businesses across the UK prepare for this transition — and the earlier you start, the more straightforward the process.

Key Benefits of SIP Trunking for UK Businesses

1. Significant Cost Savings

With SIP trunking prices starting from just £3 per channel per month, most businesses see an immediate reduction in line rental costs. Call charges are typically lower too, especially for national and international calls. There’s no need for expensive hardware upgrades if your existing PBX is SIP-compatible.

2. Scalability on Demand

Unlike ISDN, where you’re locked into blocks of channels, SIP trunks can be added or removed individually. Seasonal business? Need extra capacity for a campaign? You can scale up in minutes and scale back when demand drops — only paying for what you use.

3. Business Continuity and Disaster Recovery

Because SIP trunking isn’t tied to physical copper lines in a specific location, calls can be automatically rerouted if your office loses connectivity. Calls can be diverted to mobiles, another site, or a backup system — keeping your business reachable no matter what happens.

4. Keep Your Existing Phone System

If you’ve invested in a quality on-premise PBX, SIP trunking lets you keep it. You simply replace the ISDN connection with a SIP trunk — preserving your familiar handsets, extensions, and call-handling features.

5. Number Portability

You can port your existing geographic phone numbers (01, 02, 03 numbers) to SIP trunking, so your customers won’t notice any change. You can also easily add new numbers, including numbers from different area codes if needed.

SIP Trunking vs Hosted VoIP: Which Is Right for Your Business?

This is one of the most common questions we hear. Both SIP trunking and hosted VoIP use the internet for calls, but they serve different scenarios:

Choose SIP Trunking If:

• You have an existing on-premise PBX that you want to keep using.
• Your PBX is relatively modern and SIP-compatible (or can be upgraded with a gateway).
• You have in-house IT capability to manage your phone system.
• You want maximum control over your telephony infrastructure.
• You have a large number of extensions and a PBX that’s not yet due for replacement.

Choose Hosted VoIP If:

• Your PBX is old, unsupported, or due for replacement.
• You want a fully managed, cloud-based phone system with no on-site hardware to maintain.
• Your team works remotely or across multiple locations and needs softphone/mobile apps.
• You want features like unified communications, CRM integration, and video conferencing built in.
• You prefer a simple per-user-per-month pricing model with no capital expenditure.

For a more detailed comparison, including pricing breakdowns and real-world use cases, read our in-depth guide: SIP Trunking vs Hosted VoIP: Which Is Best for Your Business?

In many cases, the right answer depends on where your business is in its technology lifecycle. If your PBX has years of life left, SIP trunking is the sensible, cost-effective move. If you’re ready for a complete refresh, hosted VoIP may deliver greater long-term value.

What Does SIP Trunking Cost in the UK?

SIP trunking is one of the most cost-effective business telephony solutions available. Here’s a rough guide to typical UK pricing:

• Per-channel cost: From £3 to £8 per channel per month, depending on provider and volume.
• Call charges: Many providers include UK landline and mobile minutes in bundles, or offer competitive per-minute rates (often 1p–2p per minute to UK landlines).
• Setup and number porting: Often free or minimal cost when you sign up for a contract term.
• Hardware: If your PBX is already SIP-compatible, there’s no hardware cost. If not, a SIP gateway typically costs £200–£500 as a one-off investment.

To put this in context, a 10-user business moving from ISDN30 to SIP trunking could save anywhere from £2,000 to £5,000 per year, depending on call volumes and current contract terms.

What You Need to Get Started with SIP Trunking

Migrating to SIP trunking is simpler than most businesses expect. Here’s what you’ll need:

1. A reliable internet connection — A business-grade broadband, ethernet, or leased line connection with sufficient bandwidth. As a rule of thumb, each concurrent call requires around 100 Kbps of bandwidth.
2. A SIP-compatible PBX — Most modern PBX systems support SIP natively. Older systems can often be connected via an analogue telephone adapter (ATA) or SIP gateway.
3. A quality SIP trunking provider — This is crucial. You need a provider with a resilient, UK-based network, excellent support, and transparent pricing. Connection Technologies ticks all of these boxes.

The migration process typically involves configuring your PBX to register with the SIP provider, porting your phone numbers, and testing call quality. With an experienced provider managing the process, most businesses experience zero downtime during the switchover.

Why Choose Connection Technologies for SIP Trunking?

As a specialist UK B2B telecoms provider, Connection Technologies has helped hundreds of businesses transition from ISDN to SIP trunking. Here’s what sets us apart:

• Competitive pricing from just £3 per channel per month.
• Fully managed migration — we handle the technical work so you don’t have to.
• UK-based support — speak to real engineers, not call-centre scripts.
• Resilient network with built-in failover and disaster recovery options.
• Honest, independent advice — if hosted VoIP would serve you better, we’ll tell you.
• Complementary services — we can also supply your business broadband, leased lines, and mobile contracts, giving you a single point of contact for all your telecoms needs.