Can't receive calls on iPhone unless restarted, how do I fix this?

Contact your carrier. They are the ones in charge of your calls. But if they don’t find anything on their end, they may be a hardware issue with your iPhone, possible the modem. In which you would need service at your nearest Apple Store Genius Bar or Apple Authorized Service Provider.

Genius Bar Reservation and Apple Support Options - Apple

“Can't receive calls on iPhone unless restarted, how do I fix this?: i keep having to shut my phone off for a few minutes then turn it back on because people cannot call me or i cannot call out. this is happening more often. What do I do if an emergency were to arise?”

———-

Troubleshooting iPhone Cellular:

• Start here: If you can't Make or Receive Calls on your iPhone - Apple Support

• Who to Contact: Contact your Carrier, being they are the ones who provide you with the services of Cellular