ISDN (Integrated Services Digital Network) is the phone company’s 1980s answer to a modern question: how do you get digital calls down an analog copper line? It replaced the hiss of the landline with clean digital channels, and for about twenty years it was how serious businesses bought phone service. Now it is being switched off. Germany finished its migration in 2020, Australia in 2022, and the UK, the deadline most readers are up against, retires its ISDN and analog lines on January 31, 2027. If your office still has an ISDN line, you are shopping for its replacement on a deadline.
What ISDN actually is
An ISDN line splits one copper pair into digital channels. Bearer (B) channels carry the call itself at 64 kbit/s each; a Delta (D) channel carries the signaling: call setup, teardown, and caller ID. Two products mattered:
| BRI (Basic Rate) | PRI (Primary Rate) | |
|---|---|---|
| Channels | 2 B + 1 D | 23 B + 1 D (US T1), 30 B + 1 D (European E1) |
| Simultaneous calls | 2 | 23 or 30 |
| Typical buyer | Home offices, small shops | Businesses with a PBX |
Each call is circuit-switched: the network reserves a full 64 kbit/s channel for it, end to end, for the whole call. The voice inside that channel is ordinary G.711 PCM, the same codec family your calls use today, which is why decades of phone audio all sound the same.
Why businesses bought it
In 1990, ISDN was a revelation. One line carried two calls at once (or one call plus a data session that didn’t screech). PRI gave a switchboard 23 lines over a single cable, with direct inward dialing so callers could reach a desk without a receptionist. Caller ID arrived as standard. Call setup took a fraction of the analog dial-and-wait. Germany loved it most: by 2003, about 25 million ISDN channels were live there, close to a third of all subscriber lines. Radio stations built their remote studios on it, and some of that gear is still in racks today.
The catch was the price and the plumbing. ISDN needed a technician, special termination hardware, and a PBX that spoke its signaling. When broadband and VoIP arrived with more calls over cheaper lines, the case for paying carrier rates for 64 kbit/s circuits collapsed.
The switch-off, country by country
Carriers are retiring ISDN because the copper underneath it is being retired, and because the circuit-switched core network costs more to maintain than the IP network that replaced it. The dates, as of November 2026:
| Country | Status |
|---|---|
| Netherlands | KPN ended ISDN in 2019 |
| Germany | Deutsche Telekom completed the all-IP migration in 2020 |
| United States | AT&T discontinued ISDN in 2021; Verizon shut its remaining services down through 2025–2026. No single national date |
| Australia | Telstra’s final ISDN exit was May 31, 2022 |
| United Kingdom | Openreach switches off PSTN, ISDN and ADSL on January 31, 2027 |
The UK date is the one still ahead of us, and it has already moved once (from December 2025). Sales of new analog lines stopped in September 2023, and per BT’s own guidance, ISDN lines are excluded from the automatic migration program: they stop working on the deadline unless you move them yourself. Openreach warned in early November 2026 that there were fewer than 100 days left.
What replaces ISDN
The replacement is IP in every case; the choice is where the IP ends:
- A SIP trunk, if you have a PBX you want to keep. The SIP trunking guide covers the shape: your PBX keeps its handsets and features, and a provider carries calls to the phone network over your internet connection. PRI’s 23 channels become however many concurrent calls you pay for.
- Hosted VoIP (a cloud PBX or UCaaS seat), if the PBX can go too. The provider runs the phone system; you keep the numbers and the handsets or softphones.
- An ISDN-to-SIP gateway, as a bridge. It sits between the old PBX and the new trunk so legacy hardware survives the migration. A stopgap, not a destination.
The migration has two traps. First, the phone line is rarely just phones: fax machines, alarm diallers, lift phones, door entry systems, and payment terminals all found their way onto ISDN over the years, and BT’s guidance calls out exactly that list. Audit the sockets before the cutoff, not after. Second, number porting takes days to weeks. Start it early, and don’t cancel the old lines until the port completes.
Where Cartesia fits
Cartesia is not a carrier, and we don’t sell ISDN, phone lines, or dial tone. If you need those, a carrier or UCaaS provider is the right call.
Where we fit is one step further along the migration. Many ISDN lines feed a phone menu or a contact-center queue, the “press 1 for sales” flow that exists because humans are scarce. When that line moves to IP, the question worth asking is whether the menu should survive the move. A voice agent answers on the first ring, holds a real conversation, and only hands off when it should. Cartesia agents take calls on your own SIP trunk over TLS with SRTP, on numbers imported from your Twilio account, or on Cartesia-provisioned US numbers, and agent calls cost $0.03 per minute, plus LLM usage and, on Cartesia numbers, the $0.014 per minute telephony fee. If the ISDN audit above ends with a queue that shouldn’t outlive its line, the playground is the place to hear the alternative.