[Doohickie]

My son has an LG G4 on AT&T just like mine. Since the marshmallow update earlier this month, he noticed his phone doesn't work properly. What happens is that if he makes a call or if he receives a call, he can't hear the other person and the other person can't hear him. After exactly 20 seconds, the phone hangs up. All other sound and voice functions seem to be working.

Any ideas how to fix this?

[belodion]

Have you cleared the Cached Data in Settings, Storage?
If no go, try Safe Mode, in case it's third-party app trouble.
If no go, perhaps try a backup and Factory Reset.

[kojak]

Not so sure it is the phone causing you these problems. My wife's iPhone 6 plus on AT&T is acting the exact same way. Was going to exchange it but now thinking it might be a AT&T issue.