1. Check the signal strength on the phone screen, and move to an area with better reception. 
  2. Check if the issue occurs when you activate the speaker of the phone or with the wired headset. 
  3. Check if the issue is with a specific number on outgoing or incoming calls, and does it make a difference who started the call.
  4. Use the volume keys to reach the most comfortable sound level.
  5. Record an audio clip, and play the recording. This will test your microphone and speaker capabilities in one go.