Restart Your iPhone
Restarting your iPhone is a quick and easy way to fix bad call quality and a dozen other iPhone issues. Connecting to another network near you might fix the phone call problems.
How you restart your iPhone will depend on its model. If you have an iPhone with Face ID, press and hold one of the volume buttons and the side button until the power off slider appears on your screen.
If you have an iPhone with Touch ID, you’ll need to only press and hold the side button until the power off slider appears.
Move the slider to the right and let your iPhone turn off. Afterward, wait around 30 seconds before you turn on your device again.
To turn your iPhone back on, press and hold the side button, regardless of which iPhone model you have, until you see the Apple logo pop up on your screen.
After this, try making a phone call and see if that fixes the issue.