Turn Airplane Mode On and Back Off

Another way to fix your phone call issues is to use Airplane mode. When you turn Airplane mode on, your iPhone disconnects from some connectivity features, like Wi-Fi and the cellular network. 

When you disable Airplane mode again, your iPhone will search for the nearest and strongest connections available. This means that your iPhone might connect to a closer or more powerful cellular tower, allowing it to have a better connection when making a phone call.

To turn on Airplane mode, open your iPhone’s Control Center. If you have an iPhone with Face ID, swipe down from the top right corner of your screen. On the other hand, if your iPhone uses Touch ID, swipe up from the bottom of your screen.

Next, tap the Airplane mode button in the connectivity control, which is usually in the top left corner of your screen. 

Wait a few seconds and then tap the Airplane mode button again to disable Airplane mode and try making a phone call.