While searching for another solution it was to late to try this one. (https://xiaomi.eu/community/threads/call-dropping-or-caller-does-not-hear-me-anymore.49296/page-2)
I have flashed miui_MI8SE_V10.2.2.0.PEBCNXM_Offcial (File Size 2.02 GB (2,179,154,580 bytes)) and have my 3g and and H+...