I have Z740 Radiant phone. Volume on incoming calls suddenly reduced to point I can't hear caller. How can I fix this?

[AC Question]

I tried restarting the phone several times, but it didn't help. What can I do to fix? The ringtone is fine. It's the caller's voice that I cannot hear unless I turn on the speaker.

 

[B. Diddy]

Welcome to Android Central! If you can hear the caller on speakerphone or wired/Bluetooth headphones, then the phone's earpiece speaker has probably failed (or, less likely, its opening is completely blocked by debris). Contact ZTE if the phone is still under warranty.