There is no easy solution to the problem of NATs. Anyone considering VoIP deployments should be aware of the limitations they present. As proposed standard RFC, standards begin to get deployed and gain implementation experience. Invariably, problems and inconsistencies are found, and these are corrected as the draft RFC version is built. 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.