Millions of UK businesses have been making phone calls over the same underlying network, ISDN, for decades without a second thought. Until recently, it was the invisible backbone of business communications across the country. With its switch-off now complete, it’s worth understanding what it was and what has taken its place.
What is an ISDN line?
ISDN (Integrated Services Digital Network) was introduced in the UK by BT in 1986, designed to replace the existing analogue public switched telephone network (PSTN), which was struggling to keep pace with the demands being placed on it.
Rather than transmitting voice calls as continuous analogue signals, the core idea behind an ISDN connection instead converted everything into digital data. This made calls cleaner and more consistent than anything the analogue network could offer. It also means that a single physical telephone line could carry more than one type of communication at once, which was a significant leap forward at the time.
How ISDN works
ISDN works by dividing a copper telephone line into separate digital channels. The B channels are where the actual content travels; phone conversations, fax transmissions, internet data. The D channel runs alongside them, handling the behind-the-scenes work: establishing connections, ending calls, and keeping the system coordinated. This separation of signalling from content was part of what made ISDN more efficient than its analogue predecessor.
Because ISDN used digital signals from end to end, it delivered noticeably better call quality. Analogue signals degrade over distance and pick up interference from other electrical sources. Digital signals don’t suffer in the same way. For businesses relying on conference calls or fax communications, this was a meaningful practical improvement.
The ISDN connection switch off
For all its advantages over analogue, ISDN was always a transitional technology. It relied on the same ageing copper wire infrastructure as the PSTN it improved upon, and over time its limitations became harder to ignore. Bandwidth was capped, phone numbers were tied to physical locations, and scaling up required costly hardware installations. As fibre broadband expanded and internet speeds climbed, the gap between what ISDN could offer and what businesses needed grew wider each year.
Read about Red Rose’s advice for turning your phone system into a profit engine.
BT announced its intention to retire the ISTN and PSTN in 2017, setting a 2025 deadline for the full switch-off. New ISDN lines stopped being sold some years before the cutoff, and the remaining infrastructure has been decommissioned.
The technology replacing ISDN is Voice over Internet Protocol, or VoIP. Rather than routing calls through dedicated circuit-switched channels on a copper network, VoIP converts voice into digital data packets and transmits them over a broadband internet connection. This creates a system that’s cheaper to run and easier to scale.
Many businesses make the transition using SIP trunking, which connects an existing IP phone system to the public telephone network via the internet. This allows organisations to keep much of their existing phone setup while moving away from ISDN infrastructure. Others move to fully hosted, cloud-based systems that remove the need for on-site hardware altogether.
If your business hasn’t yet addressed the PSTN/ ISDN switch-off, now is the time to act. At Red Rose, we help businesses navigate the move to modern telephony. We can assess your current setup, recommend the right solution, or manage the transition from start to finish. Get in touch to find out how we can help.