If you haven’t already updated, WAIT! I’ve read several articles about a major issue in the newest iOS 12.0.1 update.
The app that’s not working? The “phone app” is shutting down or not functioning 100%. You might be able to start a call, but not be able to use other functions, like hanging up the phone. Or you can open the phone app, but not be able to dial, choose a contact, or put the call on mute.
iPhone 8 and older seem to be the versions being hit the most with this issue.
As soon as I can find more from Apple support forums, I will update this post.
Thanks for stopping by!