Hello RKDUSMC,
Thank you for posting in Apple Support Communities. We understand that you are having issues with your call quality. We would love to provide support on this.
First we would want to make sure your device is updated with the latest iOS and carrier settings. To do both follow suggested articles here:
Update your iPhone, iPad, or iPod touch
Manually update your carrier settings on your iPhone or iPad
This particular issue maybe addressed with your carrier as well because the sound quality depends on your cellular connection at the time of the call. The fact that you were able to isolate it only happening on calls says that there may be an issue with the connectivity.
If you continue to experience issues you may need to contact Apple Support: Contact Apple Support
Thank you for allowing our support community to help.
Have an awesome and productive day!