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Glossary · Numbering

What is Trunk prefix?

  • Also called: national prefix, leading zero

Reviewed by Sujan ThapaliyaLast updated

Definition

A trunk prefix is the digit or digits dialled before a national number when calling domestically — the leading 0 in most of Europe, 1 in North America. It is a dialing instruction rather than part of the number, and it is dropped when the number is written internationally.
The three STIR/SHAKEN attestation levels, which underpin how trunk prefix 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, trunk prefix included.

In practice

How trunk prefix actually works

This is the single most common source of malformed international numbers. A UK number written 020 7946 0958 becomes +44 20 7946 0958: the country code replaces the trunk prefix rather than sitting in front of it.

Some countries retain a digit that looks like a trunk prefix but is part of the national number, which is why converting numbers by rule alone produces errors and libraries exist to do it properly.

In E.164 no trunk prefix ever appears. If a stored number contains one, it will fail from outside the country.

The trunk prefix is also why copy-pasted numbers fail so often. A number copied from a European website arrives with its leading zero, works for a local reader, and fails silently for everyone else — which is why validation on input, rather than at dial time, is what prevents the support ticket.

Worked example

UK: 020 7946 0958 dialled domestically, +442079460958 in E.164 — the 0 is replaced, not kept.

Trunk prefix: common questions

Do I drop the leading zero when dialing internationally?
Yes, in almost every country that uses one. The country code takes its place, so 020 becomes +44 20 rather than +44 020.
Does the United States have a trunk prefix?
Yes, the 1 dialled before a long-distance number domestically. It doubles as the country code, which is why North American numbers look the same either way and why the distinction is easy to miss.
Why do stored numbers with a leading zero fail?
Because they are only valid dialled from inside that country. Storing everything in E.164, without the trunk prefix, is what makes a contact list work internationally.

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 trunk prefix in the product

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