Interoperability of every feature must be demonstrated and documented, and two interoperable implementations must exist. Numerous extensions are also under development. Several of these fall under the umbrella of SIP-T, or SIP for telephony, providing a way for traditional PSTN signaling messages, such as ISUP, to be carried in SIP messages. This allows SIP to be used between gateways or softswitches in a way that provides complete transparency while retaining its strengths and services. Another extension is aimed at connecting the caller to the right terminal for a given user.
Much work is also in progress on supporting infrastructure.