SIP vs ISDN – the future of telecoms


BT have announced that the kill date for ISDN and PTSN lines is in 2025. Meaning over the coming years this legacy technology will become more and more obsolete, leaving the way open for Voice Over Internet Protocol (VOIP) solutions to take its place.

It pays to know the advantages and disadvantages of each solution to best prepare you for the eventual migration away from analogue and ISDN lines. This knowledge can help you determine the best method for your business to make the switch. SIP vs ISDN


(Session Initiated Protocol)

SIP trunks send voice calls over an internet connection, this solution can be used on both digital and IP handsets meaning legacy hardware does not necessarily need to be replaced to move to SIP.

Advantages of SIP

  • Low cost calls. Often SIP calls are free of charge for national, local and mobile calls, saving up to 60% over ISDN charges.
  • International calls are also greatly reduced on SIP.
  • Increased scalability with low costs to add additional users as businesses grow.
  • Affordable monthly rentals, with calls often bundled in with the price.
  • Additional features beyond calls, such as video calling and system integration.
  • Future proof- once ISDN network is switched off VOIP will become the primary method telephony is connected by in the UK.
  • Only pay for whats needed,. With ISDN you may end up paying for additional channels that go unused.

Disadvantages of SIP

  • Quality of calls is dependant on the internet connection it runs on. Having a converged internet circuit can combat this. Converged Internet is where voice traffic has a dedicated section of bandwidth to ensure quality of service and routes directly to the SIP provider instead of over the open internet where it is vulnerable and subject to congestion.
  • No internet = No phones. If internet connectivity goes down then without a fail over service businesses will be left with no working telephones.Diagram above shows how a converged internet circuit provides quality of service for voice calls by splitting bandwidth between general internet access and the phone network. This means that even busy periods of internet use will not degrade the quality of calls.



(Integrated Services Digital Network)

ISDN has been a popular, professional grade business telephony solution for many years now, however with the development of SIP and BT’s announcement the service is to be switched off, things are starting to change. Last year alone more than 300,000 ISDN connections were lost as they migrated away to alternative solutions.

There are still several years until the cut off date and this legacy technology is not without its merits in a more digital workplace.

Advantages of ISDN

  • Crystal clear call quality that does not rely on a internet connection. Able to run even when connectivity is down.
  • Allows for multiple concurrent calls that can be scaled with the needs of the business.
  • ¬†Combines analogue and digital signals for increased quality and features.

Disadvantages of ISDN

  • Calls on ISDN are significantly more for UK numbers and mobiles than SIP alternatives.
  • International calls are very expensive to make.
  • BT will be switching off ISDN and PTSN in 2025.
  • More expensive to install and add additional lines than SIP.


Although you may have not experienced problems with ISDN, the move to SIP will need some careful consideration. Especially with the added benefits and features that are possible with VOIP solutions. The digitalisation happening across UK businesses means future proofing telephony systems is growing in importance to avoid being left behind by the ever changing industry.


Whether you are looking to make the move to SIP or are unsure of what solution is best for your business don’t hesitate to get in touch.¬† You can contact us by email at by phone at 01623 687750 or fill in the contact form at the bottom of the page and we will get back to you as soon as possible.