Can you tell if FS is involved in all calls or Kamailio dials A and B party and only sends calls to a parking lot in failure route ?
We did this long time ago with OpenSIPS where A party goes into fake ringing at a FS server and keeps dialing B party number@OpenSIPS gw every after 45 or so seconds, if B party is available FS bridges the call, if not then again stay at the parking lot. You need to set some valet_parking_ variables for timeout and orbit extension.
Given some more details I think I can help you out here.