ISDN has been the backbone of office phone systems for decades. ISDN2 gave small offices two lines on one connection, and ISDN30 gave larger sites up to thirty. Both run on the copper network that is being switched off, and 31 January 2027 is the final date to move.
If you are on ISDN today, you have three realistic options. Which one is right depends mainly on one question: do you want to keep your current phone system or not?
Option 1: Move to a hosted phone system
A hosted phone system replaces both your ISDN lines and the phone system box in your comms cupboard. The system runs in a data centre, your phones connect to it over the internet, and your provider looks after it.
Best for: businesses whose current phone system is old, out of support, or just not doing what they need. That describes most systems that are still on ISDN.
What you get:
- No hardware on site to maintain or replace
- Calls on desk phones, mobiles and laptops
- Modern features like voicemail to email, call recording and call queues
- Staff can work from anywhere on the same system
- One predictable monthly price per user
With us, that is £15 per user per month including hosting, management and support. We run hosted 3CX, hosted FreePBX and other platforms.
Option 2: Keep your phone system and switch to SIP trunks
SIP trunks are the internet-based replacement for ISDN lines. Instead of copper lines coming into your phone system, calls arrive over your internet connection. Your phones, extensions and call routing stay the same.
Best for: businesses with a relatively modern on-site phone system that already supports SIP, and that they are happy with. It is also common for larger sites, call centres and businesses with specialist integrations.
Things to check:
- Your phone system must support SIP, or be able to with an upgrade or a gateway
- Your internet connection needs enough capacity for your busiest call times
- Someone still needs to maintain the phone system on site
SIP trunks are usually cheaper than ISDN line for line, and you can add or remove channels quickly instead of waiting for new lines to be installed.
Option 3: Use a gateway to keep everything as it is
An ISDN or analogue gateway is a box that sits between your old phone system and a SIP connection, translating between the two. Your existing system thinks it is still plugged into ISDN.
Best for: very specific situations, such as a phone system that cannot be replaced yet, or a site where you need a short-term fix while you plan a proper move.
It works, but it is usually a stopgap. You are keeping an ageing system alive and adding another device that can fail. Most businesses that start with a gateway end up moving to a hosted system within a year or two.
Quick comparison
- Hosted phone system: replaces everything, lowest maintenance, most features, simple per-user pricing.
- SIP trunks: keeps your current system, replaces just the lines, good if your system is modern.
- Gateway: keeps everything as it is for now, a short-term fix rather than a long-term answer.
How to choose
Ask yourself three things:
- How old is your phone system? If it is more than seven or eight years old or out of manufacturer support, a hosted system is almost always the better choice.
- Does anyone actually like it? If your team works around its limitations, this is a good time to change.
- Do you need people working away from the office? A hosted system makes that simple. An on-site system with SIP trunks can do it too, but usually with more setup.
If you are not sure, a provider who supports both options will give you a straight answer. We do both, and we will tell you if your current system is worth keeping. Our guide to moving an existing phone system explains how a migration works.
Do not forget the other lines
ISDN often sits alongside single analogue lines for alarms, lifts and card machines. Those need to move too. See what happens to alarms and card machines after the switch-off, and our full guide to the PSTN switch-off.
Still on ISDN?
Tell us what phone system you have and how many lines. We will tell you whether to keep it or replace it, and what it will cost, on the first call.