The Retirement of ISDN – Preparing your business for the future of Telephony

With BT set to retire all its ISDN services in the UK by 2025, now is the time for businesses to explore alternatives to this increasingly dated technology.

What is ‘ISDN’ and why is it being retired?

Integrated Services Digital Network (ISDN) refers to a set of standards which enables the transmission of voice, video, audio and data digitally via the public switched telephone network. This network (the ‘PSTN’), currently constitutes a major part of the UK’s telecoms infrastructure, but due to the age of much of the equipment in the system the network is beginning to reach the end of its useful life. Additionally, many of the industry professionals responsible for the system’s deployment are nearing retirement, resulting in an ever increasing shortage of the skills required to maintain the system.

The current network will continue to be replaced with VoIP infrastructure and fibre before it’s fully decommissioned in a little over four years’ time.

How can I maintain my business’ phone system through this transition?

A few commercially available VoIP (Voice over internet protocol) alternatives to the PSTN have been around for a number of years now. The most popular and common alternative is undoubtedly SIP trunking.

SIP Trunking

In traditional telephony, the term ‘trunk’ refers to a bundle of phone lines shared by a group of people – a large office might use a trunk to connect employees to the PSTN via a private branch exchange. In VoIP terms ‘trunk’ refers virtual phonelines rather than physical cabling – however the term continues to be used for convenience.

‘SIP’ refers to the protocol used in this arrangement, and relates to the standards which define how a ‘session’ is initiated, set up and terminated.

SIP trunks can be deployed centrally or at individual branch level. When rolled out centrally all SIP trunks extend to a single location (usually a business’ head office). While this option is the easiest to establish and manage it could prove risky, as an issue with the SIP trunk could disable a business’ entire phone system until rectified. A lower risk is option is to extend individual SIP trunks to each office or branch location. This option will involve increased setup and maintenance costs but will ultimately result in a more resilient phone system.

Compared to traditional ISDN services, SIP trunks offer numerous advantages:

  • Lower costs. Internet connectivity is inherently cheaper than traditional telephony. You’ll enjoy charge-free internal calls as well as significantly reduced call charges to overseas locations. SIP users typically experience reductions in calling costs of over 50% compared to traditional telephony.
  • It’s highly scalable. SIP trunks can be added and withdrawn at any time in accordance with demand. For example, you may experience a higher volume of calls at a certain times of the year and require a greater number of lines. This characteristic makes SIP trunking more cost-flexible and adaptable than traditional telephony.
  • Use a single point of contact at any location. SIP trunks are not location-specific meaning you could for example, use your head office’s contact number at a branch office hundreds of miles away. Even if you’re planning an office move you can still retain your existing phone number to ensure easy service continuity.
  • Reduced capital expenditure. An SIP system relies less heavily on physical infrastructure than traditional telephony. Adding additional trunks may only require the purchase of additional handsets as opposed to the costly and time-consuming set up of additional phone lines. Additionally, you can make use of existing infrastructure – your existing PBX server for example may be SIP compatible.
  • Quick return on investment. Low capital expenditure and reduced call costs will result in near immediate return on investment. A breath of fresh air compared to so many technology investments which only represent value for money after months or years of use.

SIP Trunking – Key considerations

While transitioning to SIP trunking is usually straightforward, there are a few key considerations you should make before starting the process.

  • Is your infrastructure compatible? If you intend to continue using your Private Branch Exchange you’ll need to make sure it’s SIP enabled, alternatively you could migrate to a cloud-based communications platform with PBX functionality. You’ll also have to make sure that all handsets are IP compatible.
  • Can your network cope with the increased traffic? There is no doubt that migrating your business’ phone calls onto the internet will place greater strain and increase bandwidth requirements across your network. You may also want to configure your internet connection to prioritise VoIP in order to protect call quality.
  • Establish an SIP firewall. A session border controller (SBC) performs a similar function to a network firewall only it’s specifically designed to protect SIP networks. An SBC acts as a safeguard against malicious attacks but can also be used to implement wide variety of functions such as Quality of Service policing and enforcement of regulations (for example, prioritisation of emergency service calls).

Business phone systems often represent years of substantial capital investment and as a result you may feel reluctant to retire yours, but while ISDN is likely to be with us for another 4 years the alternatives already present numerous benefits over the traditional line-based systems. Making the transition to IP telephony today isn’t jumping the gun; it could prove a shrewd financial decision.



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