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There are a few things you can try:
- restart your iPhone (turn it off and on again)
- make sure your iPhone is up to date. The latest version for the iPhone 12 is iOS 14.6.
- take a look at your settings, open the app and go to General - About. If there‘s a carrier settings update, you‘ll find it there. Update via Wi-Fi.
- remove the SIM card. Make sure it‘s not damaged. If it‘s damaged, it has to be replaced by your carrier. If it looks fine, reinsert it. Maybe it‘ll work afterwards.
- go to the Settings app, open General - Reset and Reset Network Settings. This will set them back to default, you‘ll have to set up your Wi-Fi, … again.
Hope that helps.