My knowledge in this field is somewhat weak but from what I can recall Asterisk, FreeSWITCH, and Kamailio all allow for SIP communiction. FreeSWITCH and Kamaili

My knowledge in this field is somewhat weak but from what I can recall Asterisk, FreeSWITCH, and Kamailio all allow for SIP communiction. FreeSWITCH and Kamailio also support Jabber. Asterisk, it seems, has the largest developer community and support. If you're looking into using it, there is an open source version of trixbox, which should simplify the Asterisk set up and administration.