How to move from ISDN to SIP

There's many reasons to move to SIP, including BT's planned ISDN 2025 cut-off.
We'll take you through the 7 steps of the process so you can understand the journey.


Documenting requirements

The first step of the migration process involves mapping out your existing phone network, including looking at how calls flow through your company.

Things we'll look at:

  • How many phone numbers or extensions you have
  • How are inbound calls handled for every department?
  • What existing, and future, business processes need to be supported?
  • Are there any call queues or IVRs?
  • Voicemail
  • Divert features

Planning improvements

Building a picture of your ‘pain points’ is vital. This will help to identify where SIP technology can make a real difference and improve your business.

Common frustrations that SIP can solve:

High costs

Contingency measures for power outages or other unexpected disasters

Numbers being tied to one physical location

Separate systems for voice, IM, email and call records

Inability to handle temporary call volume spikes


Create a testing plan

We'll put together a testing plan, so we can make sure everything works.

This will include verifying that calls to each department are handled correctly.



We'll check your existing hardware and upgrade software or equipment to get you SIP ready.

Hardware considerations:

IP PBX or SIP gateway?

If you are already using an IP PBX, you probably won’t need to change anything. But if you are replacing an older PBX with an IP PBX it will provide greater functionality.

Switching equipment

Complete physical separation of voice and data is recommended, with a separate set of switches and cabling for SIP phones and trunking. If this is not possible, VLANs will segregate traffic.

Internet connectivity utilising SIP trunking

SIP is a network service and will use a significant amount of your available bandwidth when the lines are busy. It is common for businesses to implement a secondary connection that is dedicated to SIP services.

Security equipment

Ideally, use a Session Border Controller (SBC) to control all calls into and out of your network. Otherwise, a SIP-aware firewall should be implemented so your security equipment knows what suspicious SIP traffic looks like.


Set up

We'll make sure everything is configured to work with your new SIP service, including any new hardware.

When everything is up and running, we'll assign a few test numbers to the SIP service and run through your test plan, stress-testing any resiliency options by simulating failures.



You can now safely switch over your infrastructure to use the new SIP trunks.

The next step is a porting request for all your numbers. We’ll handle the process for you.

Porting numbers can take a several weeks, so do not cancel your ISDN lines until you can confirm all numbers are working correctly.


Cut off service to your ISDN lines

We can manage the cancellingof your ISDN accounts and shut down & decommission any ISDN-specific equipment. By this point, you should be running entirely on a modern SIP, ready to expand your communications as you grow.

Next: Start Your Move to SIP

Get started

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Contact a member of our team

We provide Business Technology solutions across the UK and will be delighted to assist you.