SIP trunking comes up in almost every conversation about business telephony in the UK right now — but most explanations either drown you in technical terminology or oversimplify to the point of uselessness. This guide explains SIP trunking in plain English, covers how it differs from other VoIP options, and helps you decide whether it is the right choice for your business.
The short answer:
- SIP trunking connects your existing on-premises phone system (PBX) to the internet to make and receive calls — replacing ISDN lines
- It is different from hosted VoIP, where there is no on-premises PBX at all
- SIP trunking is right for businesses that already have a PBX and want to replace only the ISDN lines
- Hosted VoIP (like VoIPNinjas) is right for businesses starting fresh or wanting a fully managed system
- Both options are replacing ISDN as part of the BT PSTN switch-off
What SIP trunking actually is
SIP stands for Session Initiation Protocol — the communication standard used to set up, manage, and end VoIP calls over the internet. A SIP trunk is a virtual connection between your on-premises phone system (your PBX) and the internet, replacing the physical ISDN lines that traditionally connected your PBX to the phone network.
Think of it this way:
- Traditional ISDN: Your PBX connects to the phone network via physical copper ISDN lines rented from BT. You pay monthly line rental for those lines and per-minute charges for calls.
- SIP trunking: Your PBX connects to the phone network over the internet via a SIP trunk. No physical lines, no line rental, lower call costs.
Your PBX remains in place. Your handsets, extensions, and call routing configurations remain unchanged. You are replacing only the connection from the PBX to the outside world.
How SIP trunking works
1. Your on-premises PBX is configured to use SIP instead of ISDN as its external connection
2. The SIP provider (the company providing your SIP trunks) connects your PBX to the public telephone network via the internet
3. When someone calls your business number, the call arrives via the internet to your PBX, which routes it to the correct extension
4. When your staff make an outbound call, it travels from the PBX via the SIP trunk to the SIP provider’s network, then to the destination
The caller and the recipient experience an ordinary phone call. The technology behind it is different, but the interaction is identical.
SIP trunking vs hosted VoIP: what is the difference?
This is the most important distinction for UK businesses evaluating their options.
| SIP Trunking | Hosted VoIP | |
|---|---|---|
| PBX location | On your premises | In the cloud (no PBX on-site) |
| Existing PBX required | Yes | No |
| Who manages the system | You (or your IT team) | Your VoIP provider |
| Flexibility | High (full PBX control) | High (provider manages features) |
| Upfront cost | PBX already owned | None |
| Ongoing maintenance | You maintain the PBX | Provider maintains everything |
| Best for | Businesses with a working PBX | Businesses starting fresh or replacing old PBX |
SIP trunking is right for your business if:
- You have an existing on-premises PBX that works well and you want to keep it
- Your IT team or telecoms engineer manages your phone system configuration
- You want to retain full control of call routing and system configuration on your own hardware
- You are simply replacing ISDN lines with a cheaper, internet-based alternative
Hosted VoIP is right for your business if:
- You do not have a PBX, or your existing PBX is old and due for replacement
- You do not want to manage or maintain phone system hardware
- You want your provider to handle features, updates, and system administration
- You want to scale up or down without PBX capacity constraints
VoIPNinjas is a hosted VoIP provider. We do not require an on-premises PBX. Your extensions, call routing, voicemail, and call recording are all managed through our platform — hosted in our infrastructure, accessible through a web portal and mobile app.
Why SIP trunking is becoming urgent in the UK
The BT PSTN switch-off — completing by January 2027 — is retiring ISDN across the UK. ISDN30, ISDN2e, and analogue PSTN lines are being switched off permanently, exchange by exchange.
Any business still running an on-premises PBX connected via ISDN must either:
1. Move to SIP trunking (keep the PBX, replace the ISDN connection)
2. Move to hosted VoIP (replace both the ISDN lines and the PBX)
Neither option is inherently better — it depends on whether your existing PBX is worth keeping. A modern PBX that was recently configured and works well is worth keeping and adding SIP trunks to. An old PBX that is expensive to maintain, reaching end-of-life, or under-featured is a candidate for replacement with hosted VoIP.
The costs of SIP trunking vs hosted VoIP
SIP trunking costs:
- Monthly SIP trunk channel costs: typically £5–£12 per channel (a channel is one simultaneous call)
- Call charges: varies by provider; per-minute or bundle pricing
- No PBX hardware cost (you already own it)
- PBX maintenance and configuration: your cost
Hosted VoIP costs (VoIPNinjas):
- £5.99–£24.99 per user per month (plan-dependent)
- Includes channels, call bundle, features, and system management
- No PBX hardware
- No maintenance overhead
For a business with an existing, well-maintained PBX, SIP trunking is usually cheaper on a pure cost basis. For a business factoring in PBX maintenance, end-of-life risk, and the cost of their own time managing the system, hosted VoIP often compares favourably.
What you need for SIP trunking to work
If you are considering SIP trunking rather than hosted VoIP, here is what you need:
A SIP-compatible PBX
Most PBX systems manufactured after 2010 support SIP. Asterisk, FreePBX, 3CX, Avaya, Cisco, Mitel, and Panasonic PBX systems are commonly SIP-compatible. Check your model’s specification or ask your telecoms engineer.
A stable internet connection
SIP trunks require reliable broadband. FTTC or FTTP is recommended; consumer ADSL is generally not suitable for business SIP trunking. A dedicated leased line is the highest-reliability option.
A SIP trunk provider
SIP trunk providers in the UK include VoIPNinjas, Gamma, Gradwell, VoIP Unlimited, and others. They provide the connection between your PBX and the phone network, host your numbers, and handle call routing.
A router with adequate QoS
Quality of Service (QoS) configuration on your router prioritises voice traffic over other internet usage, ensuring call quality is maintained even when your internet connection is under load.
Common questions about SIP trunking
How many SIP channels do I need?
One channel supports one simultaneous call. A business where up to five people might be on calls at the same time needs five channels. A common rule of thumb: plan for 60–70% of your total user count in simultaneous channels. So a 10-person business typically needs 6–7 channels.
Can I keep my existing business number on SIP trunks?
Yes. Your existing 01, 02, or 03 numbers can be ported to SIP trunking in the same way as hosted VoIP. The process and timescales are the same.
Is SIP trunking as reliable as ISDN?
On a quality business broadband connection, yes — and in many cases more so. Unlike ISDN, SIP can failover to a backup connection or mobile automatically. ISDN has no failover option: if the physical line fails, you have no service.
Can VoIPNinjas provide SIP trunks to connect to my existing PBX?
Yes. Alongside our hosted VoIP service, VoIPNinjas can provide SIP trunks for businesses that want to retain an on-premises PBX. Contact our team to discuss your existing setup and the options available.
Not sure which option is right for you?
Talk to our team — we can assess your existing PBX and recommend the most cost-effective path.
Start Your Free Trial — or call us: 0330 043 2388