I have need of automatically detecting whether I have called a FAX. If I am using OpenSIPS can I do this? As I understand it, OpenSIPS will get me to the point of connection then a call becomes a peer to peer arrangement meaning OpenSIPS is no longer involved and so has no way of determining anything further about the call. Is this correct?
Somehow yes, it is correct. Fax over IP works like first setting up a standard VoIP(audio) call and after that using a re-INVITE to switch to T38 codec (fax). So the call is already established when you will detect the FAX indication (by inspecting the SDP of the re-INVITE), so opensips will not to be able to do much.