I am currently experiencing reception issues with a new Murena Fairphone 4 during phone calls. On the fairphone forum I found several posts discussing the same issue. It appears that disabling VoLTE is the currently recommmended workaround, see: Losing reception all the time with FP4 - Fairphone 4 - Fairphone Community Forum or Call Microphone Issues? - #13 by NiceColdRum - Fairphone 4 - Fairphone Community Forum as well as some german posts.
Using /e/os however, It seems I am not able to actually disable VoLTE in the phone settings. Typing ‘VoLTE’ in the search bar of the settings app does recommend settings like ‘VoLTE’, ‘Advanced Calling’ or ‘4G Calling’, but tapping on them just leads to the regular mobile network tab. I also tried the phone info menu by calling ##4636## which shows that VoLTE is enabled, but I am unable to disable it in there as well. Is there any other way to to disable VoLTE in /e/os?