I can't make or receive calls, comes up call failed. I've tried everything

This worked on my iPhone 6+ with IOS 12.5.5. I'd be surprised if it works on your 12 with IOS 15. But if so...something strange is going on out there.

Try: SETTINGS >> CELLULAR DATA (turn on if off) >> CELLULAR DATA OPTION >> ENABLE LTE to OFF

Please let us/me know if that helps...

What did your carrier say when you contacted them? And what was the result when you followed the guidance in the Related Article above your name on this page, that you were reading just before you posted here? If the Related Article doesn't answer your question please post back with more details about the issue and another user in this user-to-user forum will try to help.