Please try the following to resolve your issue:
- Make sure the phone you are using supports Bluetooth 2.1 or later and A2DP to provide audio and HFP (Hands-Free Profile), will enable the microphone for phone calls and/or voice recognition.
- Check to see if the phone call is not muted.
- Check the cellular reception of the phone. A poor or weak connection could cause poor audio on a call.
- Try another phone call to make sure it wasn't isolated to one call.
- Try resetting the Headphones. For more information, see How Do I Restore The Headphones To Factory Default Settings?