The solution just work for me. Go to settings/phone settings. Remove sim card. wait for no Sim message. Do hard reset by holding the power and home button for 10 seconds. After reboot, open phone App. Check to make sure phone app is responsive. If it continues to not be responsive, go to settings/cellular and turn off all cellular data. Go back to phone app and check to make sure phone app is responsive. If it continues to not be responsive, wait for phone app to be responsive and reinsert Sim card. , turn back on Cellular data. Check phone app again for responsiveness. if your phone app is not responsive, While in the phone app, remove Sim card and wait for no Sim message to appear. Click OK and reinstall Sim card.
This is a very annoying problem but Apple needs to fix. It's called an iPhone and yet the phone part is not working… ***?
Anyways, please reply if this works for you. It worked for me.