https://datatracker.ietf.org/doc/html/rfc3398
RFC 3398 - Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation...
Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping (RFC 3398, )
isdn user partintegrated servicesdigital networkrfcisup