E.164

For each of the categories, it details the components of the numbering structure and the digit analysis required for successful routing of calls.

Annex B provides information on network identification, service parameters, calling/connected line identity, dialing procedures, and addressing for Geographic-based ISDN calls.

[1] Figure 2 E.164 numbers were originally defined for use in the worldwide public switched telephone network (PSTN).

This format includes the recommendation of prefixing international telephone numbers with a plus sign (+) and using only spaces for digit grouping.

This is called the globalized format of an E.164 number, and is defined in the Internet Engineering Task Force RFC 2806.

When a number is mapped, a DNS query may be used to locate the service facilities on the Internet that accept and process telephone calls to the owner of record of the number, using, for example, the Session Initiation Protocol (SIP), a call-signaling VoIP protocol whose SIP addresses are similar in format (user@domain...) to e-mail addresses.

This allows a direct, end-to-end Internet connection without passing through the public switched telephone network.