“Call Failed” issue after iOS 18 update

Hello, after updating to iOS 18, I'm experiencing an issue with calls. I can't make calls, including via FaceTime or Instagram, as I keep receiving a "Call Failed" message.

This problem started after the iOS 18 update, and it’s occurring everywhere—both in Prague and France.

I also received a call from an AppleCare support manager, but they couldn't resolve the issue either.

The carrier has confirmed that everything is fine on their end.

I’ve sent the phone for warranty repair, and in the meantime, I’m using an iPhone 7 without any issues.