Integrated Service Digital Network (ISDN)

Integrated Service Digital Network (ISDN) is a set of standards that carries voice and data over ordinary copper phone lines. Ratified by the ITU-T in 1988, it gave businesses a single line that could ring, fax, and transfer files at once.

ISDN was the first mainstream way to run multiple digital channels down one twisted pair, a shift Techopedia’s ISDN definition dates to the late 1980s. It replaced separate voice and modem circuits with a cleaner, faster signal.

For roughly two decades it was the default upgrade for offices that outgrew a plain telephone line but were not ready for full-fibre broadband.

Today most carriers are switching ISDN off, a retirement Ofcom has mapped in its PSTN roadmap. BT ends its UK service on 31 January 2027, and Openreach stopped selling new PSTN and ISDN lines back in September 2023.

Deutsche Telekom finished the German shutdown in 2018. Yet the protocol still runs in pockets of manufacturing, legal, healthcare, and older contact centre estates where the migration to internet-based voice has not completed.

Key takeaways

• ISDN is a circuit-switched digital standard that carries voice and data on ordinary phone wiring, originally ratified by the ITU-T in 1988.
• Two access types dominate: Basic Rate Interface (BRI) for small sites and Primary Rate Interface (PRI) for larger call centres.
• Global carriers are retiring ISDN in favour of SIP and VoIP, with the UK PSTN switch-off set for 31 January 2027.

How it works

ISDN divides one physical line into separate digital channels. Bearer or “B” channels each carry call, data, or fax payload at 64 kbps. A single Delta or “D” channel handles signalling — the dialling, ringing, and teardown steps defined in ITU-T Recommendation Q.931.

Two access types cover almost every deployment. BRI suits a home office or a small branch. PRI suits an enterprise switchboard or a contact centre floor that needs dozens of simultaneous calls.

Both use the same signalling logic, so a call started on BRI can terminate on PRI without translation.

The endpoint equipment matters. A traditional handset plugs into a Terminal Adapter, which negotiates with the carrier’s Network Termination unit at the wall.

That handshake is what lets ISDN carry a data session and a voice call on the same wire without either dropping.

Examples

ISDN survives in specific corners of the enterprise voice market. These four examples show where the standard still earns its keep and where the shutdown clock now runs loudest.

• UK legal and accounting firms. As of BT’s 2024 update, roughly two million UK business lines were still on ISDN heading into the 2027 PSTN switch-off, with regulated professions among the slowest to migrate because of records-retention rules.
• Broadcast studios. Radio journalists still use ISDN codecs for high-quality remote interviews on tight deadlines, because the fixed 128 kbps sync gives predictable audio latency that early VoIP call centre links could not match.
• Legacy contact centres in the Philippines and India. Older BPO sites built between 2005 and 2012 often ran PRI trunks into on-premise PBX gear. Many have since layered SIP on top but kept the ISDN handoff as a fallback.
• Manufacturing floors with legacy machinery. German industrial sites that missed the 2018 Deutsche Telekom cutoff moved to “all-IP” adapters that emulate ISDN signalling — a bridge Cisco has long documented in its ISDN configuration guides.

The shift is one-way. Once a site cuts over to internet phone (IP) trunking, the copper is usually decommissioned within the same fiscal year.

Related terms

Understanding ISDN means placing it against the neighbouring layers of the voice stack. The entries below run from the packet-switched successor that displaced it to the outsourcing sectors still budgeting for the migration.

• VoIP call center: the packet-switched successor that carries voice over IP networks instead of dedicated circuits.
• Internet phone (IP): the endpoint category that replaced ISDN handsets in most offices during the 2010s.
• Call center: the voice-only operation that historically depended on PRI trunks for outbound dialling capacity.
• Contact center: the multichannel evolution that made SIP-based routing the default, sidelining ISDN.
• Outsourcing: the delivery model whose telephony backbone shifted from ISDN PRI to SIP between 2012 and 2020.
• Business process outsourcing (BPO): the sector where ISDN’s retirement is now a standard site-refresh line item.

FAQ

Is ISDN still used in 2026?

Yes, but in shrinking pockets. The UK, Germany, Netherlands, and Singapore have set firm PSTN retirement dates, and most large carriers stopped selling new ISDN lines by 2020. Some regulated industries and broadcast studios still run it.

What is the difference between BRI and PRI?

BRI carries two voice channels on 144 kbps for small sites. PRI carries 23 or 30 voice channels on a T1 or E1 line for large switchboards. Both share the same signalling standard but scale differently.

What replaces ISDN?

Session Initiation Protocol (SIP) trunking over broadband replaces most ISDN lines. It routes voice as data packets across the internet, which cuts per-minute costs and adds features like presence, video, and cloud recording that ISDN cannot deliver.

Why is ISDN being switched off?

Carriers can no longer source spare parts for the ageing exchange equipment. Ofcom and equivalent regulators have set migration deadlines to consolidate national networks onto IP, freeing copper capacity for broadband.

Does ISDN still matter for outsourcing providers?

Only during migration. BPO buyers evaluating a legacy vendor should ask when the site’s PRI trunks retire, because that date drives the cost of the next infrastructure refresh.

For a plain-English tour of the technologies replacing ISDN, browse the Outsource Accelerator hubs for provider directories, buyer guides, and telephony explainers.