E.164 Telephone Numbering
Every phone number your app touches — Twilio, AWS SNS, WhatsApp, SIP — uses E.164. Getting number formatting and parsing right (libphonenumber patterns) requires understanding CC/NDC/SN structure.
What It Defines
Defines the international public telecommunication numbering plan. E.164 numbers have up to 15 digits: country code (CC), national destination code (NDC), and subscriber number (SN). The canonical format is +[CC][NDC][SN]. Used by PSTN, SIP URIs (tel:), SMS APIs, and all VoIP platforms.
Canonical (Normative)
Convenient (Practical)
Related References
The international standards body for telecommunications under the UN umbrella. Publishes Recommendations in series (E, G, H, Q, T, X…): E.164 phone numbering, G.711/G.722 audio codecs, H.323 VoIP framework, and G.9959 (Z-Wave). Free access to most Recommendations.
Related Specs
SIP is the PSTN of the internet. Every VoIP platform (Twilio, Vonage, AWS Chime, Asterisk, FreeSWITCH) speaks SIP. Building any communications product requires understanding INVITE flows, registration, and codec negotiation via SDP.
SMS is how billions of people receive OTP codes, alerts, and notifications. Understanding PDU format, encoding (GSM-7 vs UCS-2), and concatenation is essential for any platform sending SMS at scale via Twilio, AWS SNS, or direct SMPP.
ENUM is how SIP networks look up where to route a call to a phone number. Essential for VoIP providers, number portability implementations, and understanding how PSTN-to-SIP calls are routed.