E164

E.164 is the ITU-T recommendation that defines the international public telecommunication numbering plan used for the global routing of telephone calls and text messages over the public switched telephone network (PSTN) and many mobile networks. The standard specifies that an E.164 number is a globally unique sequence of digits comprising a country calling code and a national significant number, with a maximum total length of 15 digits. In practice, international numbers are written with a leading plus sign (+) to indicate international dialing, though the plus is not part of the E.164 digit string.

Structure and digits: The country calling code (CC) identifies a country or region and is 1 to 3 digits long. The national significant number (NSN) contains the area or city code and subscriber number and excludes any national trunk prefix (for example, the leading 0 used in some countries for domestic long distance). The combination CC + NSN forms the complete E.164 number and must fit within 15 digits.

Administration and use: E.164 is maintained by ITU-T within its E-series Recommendations. It underpins number allocation and dialing plans worldwide and is commonly used by routing systems in PSTN, mobile networks, and IP-based telephony contexts (for example, SIP and SMS) to carry a telephone number in a globally recognized format. E.164 is complemented by notation standards such as E.123, which define how to write numbers for international use.

Examples: United States and Canada use +1 followed by a 10-digit NSN (for example, [phone]). The United Kingdom uses +44, with the NSN length varying by region (for example, [phone]).

Notes: The total length cannot exceed 15 digits; individual countries define their NSN lengths. The E.164 framework does not define specific national numbering plans; it coordinates them to ensure global reachability.