Hi T2131,
We understand you are experiencing an issue with audio quality during phone calls.
Can we ask, does the issue occur on phone calls or FaceTime calls, or both?
We would recommend ensuring your software is up to date. You can learn how to do so here: Update your iPhone, iPad, or iPod touch.
Additionally, ensure your carrier settings are also updated. You can learn how to do so here: Update your carrier settings on your iPhone or iPad.
If the issue persists, we would recommend you Reset Network Settings. You can learn how to do so here: Return iPhone settings to their defaults.
If the issue persists further, we would recommend ensuring you have a backup of your data (How to back up your iPhone, iPad, and iPod touch), then Restore your iPhone. You can learn how to do so here: Restore your iPhone, iPad, or iPod to factory settings.
Once complete, Setup As a New Device (do not restore your backup).
If the issue is resolved, go ahead and restore your backup. You can learn how to do so here: Restore your iPhone, iPad, or iPod touch from a backup.
If the issue recurs once your backup is restored, there may be an issue with your user data, in which case we would recommend you sync your content to a service such as iCloud, then erase your device and Setup As a New Device - all synced data will return, though you will need to re-download your apps and adjust your user settings to your preference.
Hope this helps!
Thank you for using Apple Support Communities.
Cheers!