Traveler
Active Level 4
Options
Galaxy S
Sometimes phone ring volume automatically decrease.... Why?? Using Samsung s10. 
5 Comments
Sidhabhi
Options
Galaxy S
Perform reset app prefrences
Settings>applications>click at 3 dots>select reset apps prefrences
or
might be some apps are causing this cause..

As, I am not facing this issue..(S10+)

Traveler
Active Level 4
Options
Galaxy S
Will try
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Traveler
Active Level 4
Options
Galaxy S
Tried. But no result. Today faced the same issue
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sandy8160
Active Level 3
Options
Galaxy S
i am also facing same problem
Sidhabhi
Options
Galaxy S
Please run your device in safe mode and check if its happening or not,if not, then there might be some 3rd party apps causing this issue..
or
if you are using any 3rd party call recorder or keep a tab on apps like true caller
or
Turn off Ok google voice detection

just give a try

or perform normal phone reset settings