Check that you have your volume set correctly and your friend hasn't accidentally muted their end of the call.
If the audio is choppy, check that you and your friend have a good and consistent network connection. Try calling when both sides are on the cellular network, and then when both sides are on a Wi-Fi connection to see if the network bandwidth is the issue.
Try connecting a wired headset to both sides of the call (this should help a lot).
Try calling in a quieter environment to reduce background noise.
If you are trying to call another device that is physically close to your own device, a feedback loop may be causing noise. If this is the case, please try moving further away from the other device.
If you continue experiencing audio issues, please submit a request for further assistance with the following information:
You and your contact's registered phone number and email address