RFC 4715: The Integrated Services Digital Network (ISDN) Subaddress Encoding Type for tel URI

Without a tel URI parameter to carry an encoding type of Integrated Services Digital Network (ISDN) subaddress, interworking between ISDN User Part (ISUP)…

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Outgoing relationships (3 related RFCs)

Normative reference — RFC 4234: Augmented BNF for Syntax Specifications: ABNF
Normative reference — RFC 3966: The tel URI for Telephone Numbers
Informative reference — RFC 3261: SIP: Session Initiation Protocol