IOS 18.3 can no longer make or receive calls

I have an iPhone 15 Pro, and since updating to IOS 18.3, my husband and I can no longer make or receive calls. The calls incoming don’t register, and just fail going out. Easier to bulletpoint:

• Tried putting on airplane mode then off again after 15-20 secs
• Reset network settings repeatedly. This sometimes works, but for minutes only
• Been linked up to Apple support who just directed me to the help pages and asked me to do a network reset
• Updated to 18.3.1 to see if that fixed it - it didn’t
• Switched to WiFi calls only - didn’t work
• Switched off WiFi calls - didn’t work
• Checked that there are mo number blocked
• Checked all other settings on this phone but the problem persists

There are often issues with Apple updates, but this is the worst I've ever experienced since owning an iPhone (since the first one launched). I’ve completely lost faith now, and I don’t think I could have another iPhone after this. A phone that can’t make or receive calls is utterly embarrassing. Apple don’t seem to be pushing to resolve it either! I’m going to have to buy a burner phone for phone calls until this useless thing is paid up and I can move over to Android.