Is it possible to use Openser as the SIP conference server? I understand that Openser cannot do the media mixing required of a centralized media server but if it can do the centralized signaling by acting as a focus and let a media proxy do the media mixing, that would be good. I need ad hoc conferencing capability. Is there any media proxy that can achieve this together with openser?

Say A and B are in a SIP call, and A and B suddenly decide to have a conference with C. A puts B on hold and dials C' s number. After call between A and C is up, if A presses hook flash, this sends an invite to the conference application namely: conf@openseratlocalhost:5060 (which is the ip and port number at which my openser proxy would be listening). The server should then send back 200 Ok with the contact header containing the isfocus" parameter of the form -Contact: <sip:conference1@mediaproxyip;isfocus> The isfocus is an RFC 4579 based feature. The 200 Ok responses to other invites (normal calls that do not create conference) should not include this isfocus in the contact header of 200 Ok.

Finally, the openser proxy should be able to contact the media proxy on receiving an invite to create the conference and send that ip and focus information in the 200 Ok. The UA -A would then send a REFER to B and C to refer to the media server and they disconnect with A. Then all three UAs would individually send invite to the uri returned in the contact header and join the conference.