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.
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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:
↓
On-Premise PBX (Phone System)
↓
SIP Trunk (via your internet connection)
↓
SIP Trunking Provider’s Network
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Public Phone Network (PSTN / Mobile Networks / Other VoIP Users)
Let’s walk through what happens when someone in your office makes a call:
- The call is initiated — A member of staff dials a number on their desk phone, just as they always have.
- 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.
- The call travels over the internet — Voice data is converted into small packets and sent over your internet connection to your SIP trunking provider.
- 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:
| Feature | ISDN | SIP Trunking |
|---|---|---|
| Technology | Copper phone lines | Internet-based (IP) |
| Cost per channel | £15–£25/month | From £3/month |
| Call charges | Per-minute rates | Often inclusive or lower |
| Scalability | Fixed in blocks of 2 or 30 | Add/remove channels instantly |
| Number porting | Yes | Yes |
| Future-proof | Being switched off in 2027 | Long-term technology |
| Disaster recovery | Limited | Built-in failover options |
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 SIP trunking 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 pr
Frequently Asked Questions
What is SIP trunking in plain English?
SIP trunking is a way of delivering business phone calls over your internet connection rather than over old-style phone lines. Your existing on-premise phone system (PBX) connects to the public phone network via a “SIP trunk” — a virtual phone line delivered as data. Customers calling your business get exactly the same experience; the only difference is what is happening behind the scenes. The benefit is significantly lower cost (£4–£8 per channel/month vs £55–£75 for ISDN30) and easier scaling.
Do I need SIP trunking if I already have a normal phone system?
If you have on-premise PBX hardware (Mitel, Avaya, Cisco, NEC, 3CX etc) and are still on ISDN30 or analogue lines, SIP trunking will replace your line rental at a fraction of the cost without changing the phone system itself. If you are on hosted VoIP / cloud PBX, you do not need SIP trunking — the cloud service includes the equivalent. SIP trunking is the right answer when you want to keep existing PBX hardware while modernising the underlying lines.
Is SIP trunking the same as VoIP?
They are related but not identical. VoIP (Voice over IP) is the broad technology of sending voice over the internet. SIP trunking is one specific application of VoIP — connecting an on-prem PBX to the public phone network. Hosted VoIP / cloud PBX is another application — replacing the PBX entirely with a cloud service. So all SIP trunking is VoIP; not all VoIP is SIP trunking.
How much money will SIP trunking save my business?
For a typical UK business replacing 10 ISDN30 channels with 10 SIP channels: line rental drops from £550–£750/month to £40–£80/month — a saving of £6,000–£10,000/year. Call costs typically drop another 30–40% versus traditional billing. Setup is usually free if you take a 24- or 36-month term. Payback against any minor PBX configuration cost is normally well under a year.
Is SIP trunking ready for the UK PSTN switch-off?
Yes — SIP trunking is one of the two compliant replacements (the other being hosted VoIP) for ISDN and analogue lines being switched off across the UK. The PSTN switch-off completes by January 2027, after which no analogue or ISDN service will work. SIP trunking is the like-for-like replacement that lets businesses keep their existing PBX while moving to a future-proof underlying line technology.
Will SIP trunking work with my existing phone numbers?
Yes — SIP providers port-in existing UK numbers (geographic 01/02 and non-geographic 0800/03) using standard porting agreements. Porting takes 5–10 working days for single numbers, 10–25 working days for full geographic ranges. There is no charge for porting. Customers calling your existing number will not notice any change.
What broadband do I need to run SIP trunks?
Plan for around 100 Kbps per concurrent call (using G.711 codec) plus 30% headroom for jitter and overhead. A 10-channel SIP trunk therefore needs roughly 1.3 Mbps in each direction reserved — any modern fibre connection handles this comfortably. Critical requirements: low jitter (<30 ms), packet loss below 1%, and QoS configured on your router/firewall to prioritise SIP traffic over general internet traffic.
Can I add or remove SIP channels easily?
Yes — that is one of the biggest advantages over ISDN. SIP channels can be added or removed via a portal, usually within minutes, with no engineer visit. Pay-per-channel-per-month pricing means you scale up for busy seasons and back down again with no cost penalty. Compare with ISDN30, where adding capacity meant ordering an Openreach engineer and waiting weeks.
Related Guides
- how SIP trunking works in 2026
- hosted VoIP for UK businesses
- VoIP vs landline for your business
- preparing for the 2027 landline switch-off
- the PSTN switch-off explained
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