I’m not at all convinced this is a Sailfish issue… Where you get redirected during and after a call is up to the operator. For me, right now, on Telia SE it took maybe 4-5 seconds after the call finished.
One reason i can see for it to take 10s of seconds is a misconfigured network, or that you are just unlucky with the coverage overlap. I.e. you get redirected to the wrong frequency, the phone gives up on it and scans again from scratch.
Possibly this could tell you if that is the case: Cellular protocol logging with Sailfish OS
(no, i don’t even remember how to start it any more)
Then the EARFCN(s) in E-UTRAN Individual Priority Parameters in the GSM Channel Release would be different from the frequency where you eventually end up.