Not receiving incoming calls

When anyone tries to call me, they get an automated voice message saying 'the number you have dialled has not been recognised. please check the number and try again'.

My iPhone is showing good 4G signal and I can still use mobile data, and send and receive iMessages and SMS messages.

Here are two steps I have followed to try and permanently resolve the issue:

• Restart the iPhone. Once I perform the restart procedure, as listed here: https://support.apple.com/en-gb/HT201412 I can receive calls as normal again.

• Reset the 'network settings' as listed here: https://support.apple.com/en-gb/HT204027, I can receive calls as normal again.

This issue keep recurring after a couple of hours, or a couple of days (very intermittent and inconsistent) it doesn't appear to be an issue with my SIM card, but with the iPhone itself. I contacted my carrier about this issue and they suggest it is the handset too. My carrier said it is not an issue with my plan, as I am on an unlimited calls, text and data plan and have no outstanding bills to pay.

As a more drastic step, I recently set my iPhone up as a new device (not from and iCloud restore) but the problem persists. The iPhone is running the latest version of iOS at the time of writing this post (13.4.1)

I also can't make out-going calls occasionally. I get the same message as those who try to call me.

To summarise:

• When I can receive calls, I can't make out-going calls
• When I can't receive calls, I can make out-going calls

Is there a cause identified for this issue, along with a permanent fix? It's caused a huge amount of disruption and stress trying to keep in touch with family, not only for myself, but for members of my family as well.