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What is ISDN (integrated services digital network)?

  • Also called: integrated services digital network, isdn line

Reviewed by Sujan ThapaliyaLast updated

Definition

ISDN is the digital telephone standard that replaced analogue business lines from the 1980s, carrying several simultaneous calls over one connection. It is being switched off across most of the world in favour of IP, with national deadlines already passed or approaching.
The three STIR/SHAKEN attestation levels, which underpin how isdn (integrated services digital network) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, isdn (integrated services digital network) included.

In practice

How isdn (integrated services digital network) actually works

Two forms were common: a basic rate connection carrying two channels for small sites, and a primary rate connection carrying roughly thirty for larger ones.

The switch-off matters commercially rather than technically. Businesses still on ISDN face a forced migration, and the ones that plan it choose their replacement while the ones that do not get whatever is available at the deadline.

The natural replacement is a SIP trunk into the existing PBX, which keeps the handsets and the dial plan while changing the lines behind them.

The switch-off is also a commercial deadline rather than a technical one. Providers stop selling first, then stop supporting, then stop carrying — and prices rise through each stage. A business that migrates in the final year pays more for less choice than one that moved two years earlier.

Worked example

A site with two ISDN basic-rate lines carries four concurrent calls, fixed. The SIP trunk replacing it carries whatever you configure, changed in minutes rather than ordered in weeks.

ISDN (integrated services digital network): common questions

What does ISDN stand for?
Integrated Services Digital Network — the digital standard that replaced analogue business phone lines and carried multiple simultaneous calls over one connection.
Is ISDN being switched off?
Yes, on national timetables that have already passed in some countries and are approaching in others. Anyone still on it needs a migration plan rather than a renewal.
What replaces ISDN?
A SIP trunk, usually into the same PBX. That keeps the phone system and the numbers while replacing the lines, which makes it the least disruptive path.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See isdn (integrated services digital network) in the product

SIP trunking carries your phone calls over the internet to and from your existing PBX, replacing physical ISDN or PRI lines. You keep your phone system and your numbers; you drop the copper, the per-channel rental, and most of the long-distance bill.

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