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Due to the plan of the ISDN switch off, a lot of businesses are comparing the pros and cons of ISDN against its more modern counterpart; SIP (or SIP Trunks), to decide whether it’s worth switching sooner than the deadline date. (Learn more about the BT ISDN Switch Off.)

What is ISDN and SIP?

ISDN (Integrated Service Digital Network) has been the main solution for businesses for many years when it comes to their communications. It is a set of digital communication standards for the transmission of voice & data. It consists of physical copper and fibre lines connected via telephone exchanges across the world.

Session Initiation Protocol (SIP) is the set of rules that allows you to initiate and terminate a communication session, whether it’s a voice call between 2 people using an IP link. SIP trunking is the service that allows you to put this protocol into use. Essentially, SIP and SIP trunks are an on-premise solution for a telephone system that will use virtual phone lines rather than physical wires; delivered over a broadband link.

What are the Benefits and Negatives of ISDN?

Benefits of ISDN

  • It has been a proven technology for many years and is very reliable when it comes to the quality of voice calls, as it is generally delivered over a dedicated fibre or copper circuit.

  • ISDN allowed flexibility by providing multiple numbers (DDI)

Negatives of ISDN

  • You tend to be limited on phone numbers as they need to be geographical.

  • It can hinder growth plans as it does not easily expand to fit your business growth in terms of new users.

  • It can be costly if you do expand or move offices because of installation cost and also downtime would be an issue to overcome as you cannot always take numbers with you due to geographical locations, so many people have to activate call forwarding which means paying for inbound calls.

  • Limited options to keep inbound/outbound calls in service if ISDN circuit becomes unavailable.

  • High rental costs for equipment and paying for the majority of the calls.

  • Some ISDN faults can take up to 72 hours to repair.

  • Due to current technology, engineers are being trained on those first, leaving fewer engineers who are trained on ISDN phone lines.

  • All DDI numbers can only be diverted to a single number in event of an outage.

Discover the low-down on the BT ISDN Switch-off

What are the Benefits and Negatives of SIP (Trunking)?

Benefits of SIP

  • IP connectivity costs less than ISDN so you’ll benefit from lower call costs; free internal calls between sites and free calls to UK local/national and mobiles. Typically, you are looking for saving of up to 50% on line rental and 25% or more on call costs.

  • Options to choose your own phone number regardless of geographical location.

  • SIP trunking enables you to reduce the number of PBX’s you need to maintain whilst maintain your existing phone number(s) regardless if you move office.

  • SIP Trunks gives you control if you are growing (or even downsizing) and you want to retain your phone numbers. You are always in control because you can add lines, remove lines and split calls at any time.

  • Flexible as you can use it to route calls to those working remotely. Also ideal in a Disaster recovery scenario.

Negatives of SIP

  • It can be problematic if the SIP is delivered on a broadband link that is not suitable for the service.

How to make the move from ISDN to SIP

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So, which should I choose? ISDN or SIP?

The most logical solution seems to be SIP particularly with the phasing out of ISDN, beginning with not being able to purchase additional ISDN lines after September 2023 with a forced migration and switch off between April and December 2025 (see BT ISDN Switch-off blog.)

At the moment, SIP Trunking is the best alternative to ISDN as it is cheaper on a per-channel basis, it offers flexibility and control over your phone numbers and quicker to install.

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