andres,
i made another simpler cancel test. first sip:jh@test.fi invites sip:gs@test.fi, and gs starts ringing. then jh sends cancel to r-uri sip:gs@test.fi, which ser.cfg immediately at the start of the script t_relay()s out, i.e., without doing anything to the request. the result is that ser correctly sends a cancel to the ip address/port of gs.
my conclusion of this is that cancels don't need to be processed by ser.cfg, just relayed out. perhaps others familiar with tm mystics can confirm or deny this.
-- juha