If your business is still paying for traditional ISDN or PSTN lines, you’re almost certainly paying more than you need to. SIP trunking is one of the quickest and most reliable ways to cut your monthly phone bill — often by 30 to 40% — without changing how your team makes and receives calls.

In this guide, we’ll break down exactly how SIP trunking works, why it’s cheaper, and how to make the switch without disruption.

What Is SIP Trunking?

SIP stands for Session Initiation Protocol. A SIP trunk is a virtual phone line that connects your existing phone system (PBX) to the public telephone network via the internet, replacing the physical copper lines you currently use.

Instead of paying BT or another carrier for a set number of physical lines, your calls are routed digitally over your broadband connection. You only pay for the capacity you actually need — and the per-minute rates are significantly lower.

Why Is It So Much Cheaper?

There are several reasons SIP trunking is cheaper than ISDN or PSTN:

1. No line rental on unused channels

With ISDN, you pay for a fixed number of channels whether you use them or not. SIP trunks are flexible — you scale up or down based on real usage.

2. Lower call rates

SIP calls — especially to other UK numbers and mobile numbers — typically cost a fraction of traditional rates. UK landline calls can be as low as 0.5p per minute. International calls show even bigger savings.

3. Consolidate multiple sites onto one trunk

If your business has multiple offices each with separate phone systems and line costs, a centralised SIP trunk with a hosted or on-premise PBX can serve all sites from a single contract.

4. No hardware upgrades required (usually)

Most modern PBX systems — including popular platforms like 3CX, Yeastar, and Asterisk — support SIP trunking directly. You may not need to replace any hardware at all.

Real-World Example: The 40% Saving

Here’s a straightforward example based on a real SME scenario:

Cost	ISDN30 (current)	SIP Trunking
Monthly line rental	£180	£40
Call costs (monthly)	£220	£90
Maintenance & support	£60	£20
Total per month	£460	£150

That’s a saving of £310 per month — or £3,720 per year — for a business making moderate use of 10 concurrent lines. Savings scale with the size of your operation.

Is SIP Trunking Right for Your Business?

SIP trunking works best when:

• You have a reliable broadband connection (fibre broadband is ideal)
• Your existing PBX supports SIP (most modern systems do)
• You make a reasonable volume of outbound calls
• You want flexibility to grow without paying for unused capacity

It works across virtually any industry — from small professional services firms to multi-site retail and logistics businesses.

What You Need to Get Started

1. A compatible phone system

Your current PBX needs to support SIP. Most systems manufactured in the last 10 years do. If yours doesn’t, it’s often cheaper to upgrade to a modern PBX at this point rather than continuing to pay ISDN rates.

2. A reliable internet connection

Voice quality depends on your broadband. A fibre connection with sufficient upload bandwidth is strongly recommended. As a rough guide, allow around 85Kbps per concurrent call.

3. A SIP trunk provider

You’ll need a provider to supply the SIP trunks and manage call routing. Pricing, contract length, and included minutes vary — so it pays to shop around or work with a specialist who can compare options for you.

4. A Quality of Service (QoS) configuration

On your router, voice traffic should be prioritised over other internet traffic. This prevents call quality dropping when someone’s downloading files or in a video meeting. Most modern business routers support this; your provider should help you configure it.

Common Concerns — Answered

Will call quality suffer?
Not if the setup is done properly. With a good broadband line and correct QoS settings, SIP call quality is indistinguishable from ISDN — and often better.

What happens if the internet goes down?
A good SIP provider can route incoming calls to mobile numbers or alternative numbers automatically during an outage. This failover can be configured in advance so there’s no gap in service.

Is it complicated to switch?
The transition is simpler than most businesses expect. Your existing numbers can be ported across with no interruption to incoming calls. A good provider manages the entire porting process for you.

What about the PSTN switch-off?
BT Openreach is retiring all legacy PSTN and ISDN lines by January 2027. If you haven’t started the transition yet, now is the time — moving to SIP trunking now means you’re ahead of the deadline and start saving immediately.

How to Make the Switch

1. Audit your current spend — get a copy of your last 3 months of phone bills and identify exactly what you’re paying for lines, calls, and maintenance
2. Check your PBX compatibility — confirm whether your system supports SIP natively or needs a gateway adapter
3. Test your broadband — check upload speed and latency during your busiest calling periods
4. Get a comparison quote — compare SIP trunk pricing from two or three providers, looking at included minutes, per-minute rates, and contract terms
5. Plan the migration — agree a go-live date, port your numbers, and test before cutting over

Most migrations are completed in 2-4 weeks with minimal disruption.

The Bottom Line

SIP trunking isn’t new technology — but it remains one of the most underutilised cost-saving opportunities for UK businesses. With ISDN lines being switched off in January 2027 anyway, the question isn’t whether to move — it’s how quickly you can start saving.

A 40% reduction in your phone bill is realistic for most businesses. For many, the savings are even greater.

If you’d like to find out exactly how much your business could save, get in touch with the Just Business Phones team — we’ll carry out a free audit and produce a personalised comparison for you.