After each troubleshooting step, check to see if your issue is fixed.
Look for 

Clear the data for the Phone app.
Go to Settings > Apps & notifications > See all # apps > Phone > Storage > Clear storage.
Outbound calls
Incoming calls
If you have a signal and can make calls, but callers tell you they consistently get a busy signal when calling you, check that you haven’t blocked their numbers.
Check your Do Not Disturb settings:


