State of a single call, threaded through ExKamailio.CallHandler callbacks:
the SIP identifiers plus every SDP that crossed the wire, in both
directions. A field is nil until the call has reached the step that fills it.
call_id,from_tag,to_tag: SIP identifiers forwarded by Kamailio.from_offerer_sdp,from_answerer_sdp:%ExSDP{}received from that peer.to_answerer_sdp,to_offerer_sdp:%ExSDP{}the handler returned, forwarded by Kamailio to that peer.