Calls stop unsystematically

Hi all,
I did not find a solution in the other threads covering similar problems, so I’m opening a new one:
My mother’s Fairphone 4 (running on latest Android 13) is annoying her a lot by oftenly interrupting her calls after a short (varying) period of time. (Sometimes it doesn’t interrupt at all, mostly anywhere near ten minutes, but not on a regular basis.)
This is happening in handsfree-mode too. (I therefor exclude the nearfield-sensor.)
It doesn’t heat up extraordinarily.
I already installed and tried another Phone-App (Fossify Phone via F-Droid.)
My mum is living next to Frankfurt (no Angels nearby) and doesn’t want to send her phone in, as she is using it daily and doesn’t have a replacement phone.
Do you have anymore suggestions?
Greetings from Kiel
Boris