Session Initiation Protocol (SIP) is an endpoint-oriented messaging standard defined by the Internet Engineering Task Force (IETF). SIP trunking functionality is available on any Linux-based server. Linux servers function as Plain Old Telephone Service (POTS) gateways. These servers support name and number delivery among the various non-SIP endpoints, such as analog, DCP, or H.323 stations, and analog, digital or IP trunks that Communication Manager supports. These servers also support name and number delivery between SIP-enabled endpoints, such as the Avaya 4600-series SIP Telephones. In addition to calling capabilities, IP Softphone Release 5 and later include optional instant messaging client software, which is a SIP-enabled application. IP Softphone Release 5 also continues full support of the existing H.323 standard for call control. Avaya SIP Softphone Release 2 and later release fully support SIP for voice call control, instant messaging, and presence.

Communication Manager assigns two types of numbering to an incoming SIP trunk call:

Public and private numbering plans are important when the incoming SIP trunk call is routed back over an ISDN trunk group.

ISDN defines numbering plans (NPI) and types of number (TON) within those plans.

If the caller does not know or does not want to specify the TON or NPI, Communication Manager can set that value to Unknown. When an incoming SIP call is routed to an ISDN network, Communication Manager always sets the TON to Unknown.