I don't think there is a solution.
I have this problem on an iPhone 7 as well. FaceTime connects, then hangs up with a "Call failed" message. As it's a spare phone, I completely wiped it and set it ip again with a brand new Apple ID. Still the same problem. The same Apple ID that fails can make FaceTime calls using another iPhone (an X in this case).
The problem started in iOS 14. Updating to iOS 15 and then 15.1 did not make one bit of a difference.
So it's not the installed software or a setting. It's also not the Apple ID. I also don't have a case on my iPhone 7 so there's no magnet or anything.
Maybe connecting the iPhone 7 to a computer with Xcode allows me to see a crash log of some kind.
Is it some
Iemand
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macOS 11.4, Safari 14.1.1, iPhone X, same issue.