A big step to converge various IM/VoIP networks world wide has been done with the new XMPP module, developed by Andreea Spirea. It allows exchange of instant messages with any XMPP network out there (like Google Talk or Jabber), opening the way to add presence and voice support in the near future.

There is no requirement of mapping SIP addresses to XMPP addresses via database of other persistent storage, the addressing schema allows translation on the fly. Just install the SIP-to-XMPP gateway and all your SIP users become available in the XMPP network and your users can chat with anybody in XMPP world. You can be even a SIP-to-XMPP relay for SIP networks you peer with, there is no limitation that only local users can use the gateway.

The conversation will survive to restarts, the session being recovered form the messages. As a result, there is no need of a persistent storage, the footprint is very small, embedding the gateway in small devices should be straightforward.

For more technical details see:

http://www.openser.org/docs/modules/1.2.x/xmpp.html