SIP Trunk Providers and Your PBX: Compatibility Guide

Most SIP trunk providers will tell you their service works with “any PBX”. In practice, that’s only partly true. Whether your system connects cleanly depends on its age, its software, how it handles calls and what else is plugged into it.
The question is more pressing than it used to be. The UK’s legacy phone network is due to be retired on 31 January 2027, and Openreach has said it will not be delayed again. Firms with a working PBX on ISDN lines often want to keep it, and SIP trunking is the usual way to do that.
This guide shows how to check compatibility before you sign, so surprises happen on paper rather than on cutover day.
Three Types of PBX, Three Different Answers
Modern IP PBX
Most systems installed in the last decade speak SIP natively. They connect to a trunk over your network with no extra hardware. Compatibility problems here are usually about settings and licences, not capability.
Older Digital or TDM PBX
Systems built around ISDN or proprietary digital handsets don’t speak SIP. They can often still work through a gateway, a small device that converts ISDN signalling into SIP. It adds cost and a point of failure, but it can extend the life of a system that still does its job.
Hosted or Cloud PBX
If your phone system is already hosted, you don’t need a separate trunk for it. Your provider handles the connection. SIP trunks come into play only if you’re keeping or adding an on-site system alongside it.
If you’re unsure which you have, ask your PBX supplier for the make, model and software version. Everything else in this guide depends on those three facts.
The Compatibility Checklist
1. Does the PBX Support SIP Trunks at All?
Check the manufacturer’s documentation for “SIP trunk” support, not just SIP handsets. These are different features. Some older systems support SIP extensions but not external trunks.
2. Are Trunk Licences Required?
This is a classic hidden cost. Some PBX vendors charge per SIP channel, so a twenty-channel trunk may need twenty trunk licences on your system. Ask your supplier what you already own and what you’d need to buy.
3. Is Your Software Up to Date?
Old firmware can cause odd faults, such as dropped calls, one-way audio and failed transfers. Check whether your version is still supported and whether upgrading first makes sense.
4. Which Codecs Does It Use?
Codecs compress voice for transmission. G.711 is the common baseline in the UK and is widely supported. Others, such as G.729, use less bandwidth but may need licences or cause quality trade-offs. Make sure your PBX and the provider share at least one codec, ideally G.711 as a fallback.
5. How Does It Handle Touch-Tones?
When callers press keys to navigate an auto attendant, those tones must pass through correctly. There are different methods for sending them over SIP, and a mismatch causes menus that ignore keypresses. Ask your provider which method they use and confirm your PBX can match it.
6. How Does Authentication Work?
Providers usually authenticate your PBX in one of two ways: registration with a username and password, or IP-based authentication, where your public IP address is approved. Your PBX and network must support whichever the provider uses, and IP-based methods need a fixed public address.
7. What About Your Firewall and Router?
SIP traffic passes through firewalls and routers that sometimes mangle it. A feature called SIP ALG, built into many routers, is a frequent cause of one-way audio and dropped calls, and is often switched off for that reason. Confirm how your network is configured, and whether your provider recommends specific settings or a Session Border Controller.
8. How Are Numbers Presented?
ISDN systems often received numbers in a particular format, such as the last four digits of a direct dial. SIP providers may send the full number in national or international format. The PBX must be configured to match, or inbound calls reach the wrong extension or none at all. Test this early, because it’s one of the most common cutover problems.
9. What About Caller ID?
Check which number your outbound calls display and whether your PBX can set it correctly per user or department. Mistakes here can confuse customers, and presentation rules for numbers matter, so ask the provider what they check.
10. Does Encryption Matter to You?
Some firms need encrypted signalling and media for security or policy reasons. If so, check that both your PBX and the provider support it, and ask how it’s configured.
Fax, Alarms and Other Awkward Devices
PBXs often have devices attached that don’t behave well over SIP.
Fax machines. Fax over standard SIP can be unreliable. Providers may support a dedicated fax method, or you may be better off using an online fax service.
Alarms and monitoring lines. Many use old phone lines and tones that SIP doesn’t carry reliably. Ask each alarm supplier for a digital alternative.
Lift emergency lines, door entry and card machines. These often depend on analogue lines, and each needs its own plan.
Analogue phones. These can connect through an analogue adapter, but you should test them.
Doing this audit early, before you commit, avoids last-minute scrambles.
Gateways: When You Need One
If your PBX is ISDN-only, a gateway is the bridge. Ask:
Who supplies, configures and supports it?
How many channels can it handle?
Does it have a backup or failover option?
Is it a one-off cost or rented?
The gateway becomes part of your phone system, so treat it that way. A cheap unit with no support can become your weakest link.
How to Judge Providers on Compatibility
When comparing SIP trunk providers in the UK, don’t accept “works with any PBX” as an answer. Ask for specifics.
Have you connected my PBX make, model and software version before?
Can you share a configuration guide or reference customer?
What are your settings for codecs, touch-tones, authentication and number format?
Will you test before we port numbers?
What support do you give if the problem is in my PBX, not your platform?
You’ll find plenty of online lists of the best UK VoIP providers, and plenty of buyers searching for the cheapest UK VoIP providers. Neither tells you whether a given SIP provider has actually connected to a system like yours. A cheap SIP trunking service that doesn’t work with your PBX costs far more in the end than a slightly pricier one that does.
Test Before You Port
The safest approach is to run a trial before moving your main numbers.
Set up a test trunk with a few channels and a spare number.
Make inbound and outbound calls across every route you use.
Check touch-tones, transfers, voicemail and call forwarding.
Check number presentation and caller ID.
Test fax and any other devices.
Test failover, such as what happens when your internet drops.
Only then port your main numbers, keeping the old lines live until you’re confident.
A Realistic Example
A 20-person logistics firm in Birmingham has a PBX installed seven years ago and an ISDN30 line. The supplier says it supports SIP, so the firm assumes the switch is straightforward.
During a test trunk, they find three issues: the PBX needs additional trunk licences, inbound numbers arrive in a different format and the old fax line fails. They buy the licences, adjust the number settings and move fax to an online service. Cutover day passes quietly because the problems were already fixed.
Common Mistakes
Assuming “SIP-capable” means “trunk-capable”. Check the documentation.
Forgetting licenses. They can add real cost.
Skipping the number format test. It’s a leading cause of misrouted inbound calls.
Ignoring the router. SIP ALG alone has caused many mystery faults.
Not testing fax and alarms. These fail quietly until someone needs them.
Porting before testing. Always trial first.
Wavetel Business is one example of a UK provider you might speak to about compatibility, and any provider offering business SIP trunking should be willing to review your PBX details and put their recommendations in writing.
Frequently Asked Questions
Will my existing PBX work with SIP trunking?
Often, yes. Modern IP PBXs usually support SIP trunks directly, while older ISDN systems usually need a gateway. Confirm your make, model and software version with your supplier and provider.
Do I need a gateway?
If your PBX only supports ISDN, yes. A gateway converts ISDN signalling into SIP. If your PBX already supports SIP trunks, you don’t.
Why would my PBX need extra licences?
Some vendors charge per SIP channel. Ask your supplier what you already have and what a new trunk would require.
Can I keep my existing numbers?
In most cases, yes, through number porting. Test the trunk first, then port, and keep the old lines live until you’re confident.
What happens to my fax machine?
It may not work reliably over standard SIP. Ask your provider about fax support, or consider an online fax service.
Conclusion
SIP trunking can extend the life of a good PBX, but only if the two actually work together. The main things to check are SIP trunk support, licences, codecs, touch-tones, number format, router settings and the devices hanging off your system.
The practical takeaway is to gather your PBX details, ask providers specific compatibility questions and run a test trunk before porting any numbers. Start well before January 2027 so any problems come to light in time to fix them.