SIP trunks vs ISDN: what actually changes
A straight comparison of SIP trunks and ISDN lines: how channels work on each, what you can change and how fast, what it costs, and which businesses should care.
Published 8 April 2026 · updated 20 August 2026
If you already have a phone system you are happy with, you may not need to replace the whole thing. You may only need to replace the lines feeding it.
That is the SIP trunk conversation, and it is worth understanding before anyone quotes you for a full rip and replace.
The same job, different plumbing
A trunk is the connection carrying calls in and out of your business. ISDN did it over a dedicated physical circuit. SIP does it over the internet connection you already have.
Everything else about how your phone system behaves internally stays conceptually the same. Extensions, transfers, hunt groups, hold music. The difference is entirely in how calls reach the outside world.
The difference that matters: channels
On ISDN, the number of simultaneous calls is fixed in hardware. An ISDN2 carries two. An ISDN30 carries thirty. Want more? Order another circuit, wait weeks, pay for installation.
On a SIP trunk, the number of channels is a setting on the provider’s platform. Five, twelve, forty, whatever matches your actual usage. Changed remotely, usually the same day, with no engineer visit and no new cable.
For a seasonal business this is the whole argument. A firm that needs thirty channels for six weeks a year and eight for the rest has been paying for thirty all year long, forever.
What it costs
ISDN was expensive because a dedicated circuit is expensive to install and maintain. SIP rides on broadband you are already paying for, so the cost drops to the service itself.
The practical saving for most businesses comes from three places: no separate line rental, channels matched to real usage instead of hardware capacity, and cheaper call rates.
What SIP needs from you
A stable internet connection with enough headroom. Voice is not bandwidth-hungry, roughly 0.1Mb per call in each direction, but it is sensitive to jitter and packet loss. A connection that is fast but inconsistent will produce calls that break up.
Any provider worth using will test this before quoting rather than after installing.
The honest limitation
If the internet goes down, so do the calls. On ISDN, the circuit was separate from your broadband, so a broadband fault did not touch the phones.
The mitigation is call forwarding: a rule that automatically sends calls to mobiles if the office connection drops. It works well, and callers generally do not notice. But it is a mitigation, not a guarantee, and anyone claiming SIP has no downside is overselling.
Should you keep your existing phone system?
Sometimes. If your PBX is relatively modern and supports SIP, swapping the lines underneath it is cheaper and less disruptive than replacing everything.
But be realistic about it. If the box is ten years old, out of support, and every change requires an engineer visit and an invoice, keeping it means keeping all of that too. The line replacement fixes your connectivity and leaves your actual problem untouched.
For most small businesses we look at, the honest answer is that a hosted system at £10 a phone with everything included works out cheaper than keeping an old PBX alive on new lines. But we will tell you when it does not.
The deadline
ISDN2 and ISDN30 both stop working on 31 January 2027 along with the rest of the copper network. This is not a soft deadline or a support cutoff. Full detail here.
If you are on ISDN and have not started this conversation, now is a considerably better time than next autumn, when every business in the country will be trying to move at once.