[Serusers] fr_inv_timer

hi all, I have modparam("tm", "fr_inv_timer", 60) when a callee doesn't anwer, he often send a 487 (cancelled) before 60 seconds... It means timeout in fact or no answer... If the caller stop the call, I also get a 487... How to make the difference on a 487 generated by the caller and by the callee? It makes difference, if it's generated by the callee, I will forward the call to voicemail or another number, if it's generated by the caller, I will do nothing. When I set fr_inv_timer to 15, I always get a timeout and it's ok, but my customers dislikes that as well as the callees who needs to be better than Ben Jonhson for the 100meters :( Does exists a parameter to be send to the callee to tell him to wait 60 seconds + 1 before sending me a Cancel? Regards, Olivier