Skip to content
AI Dialer

Glossary · Voice technology

What is PRI (primary rate interface)?

  • Also called: primary rate interface, pri line, t1 pri

Reviewed by Sujan ThapaliyaLast updated

Definition

A PRI is the higher-capacity form of ISDN, carrying roughly twenty-three simultaneous calls in North America or thirty in Europe over a single circuit, plus a channel for signalling. It was the standard connection for a business PBX before SIP trunking.
The three STIR/SHAKEN attestation levels, which underpin how pri (primary rate interface) 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, pri (primary rate interface) included.

In practice

How pri (primary rate interface) actually works

Capacity is fixed and physical, which is the defining limitation. Needing one more concurrent call than the circuit supports means ordering another circuit and waiting weeks, where a SIP trunk adds a channel in software.

PRIs are reliable and predictable, which is why some sites kept them long after the alternatives matured — the failure modes are well understood and independent of the internet connection.

As ISDN is withdrawn, PRIs are being retired with it, and the replacement path is the same: a SIP trunk into the existing PBX.

PRIs also fail in a way that is easy to miss: the twenty-fourth call simply does not connect. There is no degradation, no warning and nothing in the phone system to indicate capacity was the cause, which is why sites regularly discover the limit through a customer complaint rather than a monitor.

Worked example

A 23-channel PRI carries 23 concurrent calls. A team making 800 short calls a day rarely exceeds a dozen at once, which is why concurrency rather than volume decides the sizing.

PRI (primary rate interface): common questions

How many calls can a PRI carry?
Twenty-three simultaneous calls in North America, thirty in most of Europe, plus a separate signalling channel. That capacity is fixed by the circuit.
Is a PRI better than a SIP trunk?
It is more predictable and far less flexible. SIP trunks add capacity in software and cost less; PRIs fail independently of your internet connection, which is the only remaining argument for them.
What happens to my PRI when ISDN is switched off?
It is withdrawn on the same timetable. The migration is normally a SIP trunk into the same PBX, which keeps handsets, numbers and dial plan intact.

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 pri (primary rate interface) in the product

A hosted PBX is a business phone system that runs as software in a provider's data centre instead of a box on your premises. Handsets and softphones register to it over the internet, and capacity, extensions and numbers change in configuration rather than by an engineer visit.

Explore hosted pbxGet early access