The lowest in-call volume 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