The lowest in-call volume level I have a redmi 4x, lineage os 15 official.
I have the lowest in-call volume level as big as 20%. I can't make it lower and in a quiet room almost everyone can hear my phone conversation. I tried different apps, some of them worked: they could lower the volume to 0% but the effect was temporal. I'm looking for a constant solution. Can anybody suggest a method how to lower the in-call volume below that level?
The cause is Volume Steps are set to 5 ... At 20% you must have only 5 steps and 20% per step... You need to modify your build.prop and add more in-call volume steps.. Here's an example of 20 steps... 20/100 = 5% volume as minimum..

Soo...
If you MUST Have a minimum of 0 or 1% then set the steps to 100... I recommend 20 however..   

Add to build.prop


Go to Development Options
Enable Root Access for Apps
Open a root explorer 
Open system / build.prop 
Edit this line if it exits, or add it if it doesn't...   

ro.config.vc_call_vol_steps=20