So I asked a friend to download Softphone for screendumps of all screens to write a manual for remote guests.
It occured to me that in the ‘advanced audio settings’ the sampling frequency is only 16000Hz.
Shouldn’t this be set to at least 44100 or 48000? Is this a system wide setting within Android?
This explains me why an Android caller still sounds like a phoneline instead of clean VOIP when only the smartphone is used.