VoIP glossary
What is E.164?
E.164 is the international standard for phone number format: a country code followed by the national number, up to 15 digits in total, with no leading zeros or trunk prefixes.
E.164 explained
In E.164 a London number is written +44 20 7123 4567 rather than 020 7123 4567, and a US number is +1 212 555 0100. On SIP trunks the plus sign is often dropped, so the same numbers are sent as 442071234567 and 12125550100. Using one consistent format everywhere removes a whole class of routing errors.
Most "number not found" (404) and "address incomplete" (484) failures come from numbers that are not in E.164: a missing country code, a national 0 left in, or an international 00 prefix the carrier does not expect. Normalise numbers on your PBX before they reach the trunk.
Example