Consider Wi-Fi Calling

If you’ve tried everything and your iPhone’s regular phone calls aren’t working, you might as well try your iPhone’s Wi-Fi Calling feature. This will not fix your regular cellular phone calls, but it will let you make or receive phone calls with a Wi-Fi connection. 

You can only use Wi-Fi calling if your carrier supports it. All the major US carriers do, but you may have problems if you’re on a smaller carrier or an MVNO. The feature should only appear on your iPhone if it’s available. Here’s how to find it and switch it on:

1. Open the Settings app.
2. Tap on Cellular.
3. Go to Wi-Fi Calling.
4. Turn on Wi-Fi Calling on This iPhone.