The ITU-T standard that defines the international telephone numbering plan, with numbers of up to 15 digits made up of a country code and a national number.
E.164 is the format that makes global telephone numbers unambiguous. Each number begins with a country code of one to three digits, followed by a national number, for a total of no more than 15 digits. In written form, numbers are often shown with a leading plus sign, such as +1 followed by a ten-digit North American number.
In SIP networks, numbers may appear in many formats: national, international, with or without the plus sign, or with local dialing prefixes. Normalizing numbers to E.164 makes routing, analytics, and blocking consistent. Doing this once, consistently, at the point where traffic enters the network avoids repeating the work in every downstream system.
STIR/SHAKEN relies on canonical number formats. The numbers in the PASSporT must match the numbers in the SIP signaling after normalization, so inconsistent formatting between signer and verifier can cause otherwise valid signatures to fail. Common culprits include a leading plus sign that one side includes and the other strips, or a national prefix added by intermediate equipment.
Why it matters to carriers
Inconsistent number formats quietly break things: blocklists miss matches, analytics split one caller into several identities, and verifications fail. Normalizing every calling and called number to E.164 at your network edge is a small change that improves fraud detection, DNO blocking, and STIR/SHAKEN reliability at the same time.