After each troubleshooting step, check to see if your issue is fixed.
Check your proximity sensor.
Dial a call and set the phone on a table (don't use speaker mode), then hold your hand over the proximity sensor to see if you can turn the screen off and on.
Check your signal strength:
Turn your phone off and then on again.
Press and hold the Power button, then touch Power off. Then turn it on to establish a new connection with the network.
Check that nothing is covering the microphone:
Check to see if your phone is using Voice privacy. (This feature is not available on all models.) Turn it off to see if sound quality improves:
If the issue occurs while using the speakerphone or a Bluetooth headset, check your signal strength.
In the status bar, see how many bars your network connection has. If you see only one or two bars, change locations to improve your signal strength, particularly if you are indoors.
Turn your phone off and then on again.
Press and hold the Power button, then touch Power off. Then turn it on to establish a new connection with the network.
Turn off Voice Match.
Saying “OK, Google” won't work from all screens, but you can touch the search widget first, then say “OK, Google”.
Start by checking your volume setting. You can only adjust the call volume with the Volume buttons while you’re on a call.
Wired headset
Earpiece
Check that nothing is covering the microphone:
Turn your phone off and then on again.
Press and hold the Power button, then touch Power off. Then turn it on to establish a new connection with the network.
Check battery level of Bluetooth device and phone, and charge if necessary. Bluetooth devices may disconnect or create noise when the phone or Bluetooth battery are low.