ITU-T Recommendations
ITU-T
E.164 phone numbers, G.711/G.722 audio codecs, and H.323 all live in ITU-T series — foundational for telephony.
What It Is
UN telecommunications standards body. Recommendations organized in letter series (E, G, H, Q, X, Y…) covering numbering, transmission, multimedia, signaling, and data networks.
Taxonomy
Letter series A–Z · Study Groups · Recommendation number
Top-Level Categories
Master Catalog
Sub-Indexes & Guides
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.
View all bodies →Curated specs from ITU-T
H.264 remains the most widely supported video codec. Distinguish codec (H.264 bitstream) from container (MP4/WebM) when debugging playback, transcoding, and MSE SourceBuffer codec strings.
G.729 remains common on legacy SIP/PBX trunks where bandwidth is constrained. License/patent history affected adoption — know it when debugging codec negotiation on older enterprise systems.
G.726 appears in older VoIP gateways and recorded WAV files. Often confused with G.711 — verify fmt chunk bit rate and RTP payload type mapping.
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.
H.323 remains in enterprise video conferencing (Cisco, Polycom legacy endpoints) and some carriers. Understanding it is necessary when integrating with legacy infrastructure or gateways.
G.711 is the common denominator of telephony. Every PSTN gateway and legacy SIP trunk speaks G.711. When codec negotiation fails and you're left with 64 kbps PCM, this is why.
G.722 is the standard for HD voice in SIP and enterprise telephony. When you enable 'HD Voice' on a SIP trunk, G.722 is typically what's negotiated. Essential for enterprise UC deployments.