Documentation https://www.kamailio.org/docs/modules/devel/modules/tm.html#tm.p.fr_timer says:

Timer which hits if no **final reply** for a request or ACK for a negative INVITE reply arrives (in milliseconds).

Looks like it is not correct description - fr_timer value works until 1xx(not **final**) response received and then timer restarted with fr_inv_timer value there:

https://github.com/kamailio/kamailio/blob/master/src/modules/tm/t_reply.c#L2...

Looks like old doc from sip router https://sip-router.org/wiki/ref_manual/timers explains it correctly:

fr_timer

This timer is used for all SIP requests. It hits if no reply for an INVITE request or other request has been received (F in milliseconds). If a provisional reply is received for an INVITE (any 1xx), then the fr_inv_timer will be used instead. And if no replies (at all) for an INVITE are received before `fr_timer` hits, the transaction is terminated with a 408 in failure route.