What exactly does that mean?
Is the phone ringing, but there are no buttons displayed to accept or reject the call?
A somewhat common cause of this across many different phones would be Settings - Apps & notifications - See all - Phone - Notifications … could be off in general or could be off in detail for incoming calls.
Else … Do Not Disturb Mode could be activated. Is there something that looks like a “No entry” sign in the status bar?
Or don’t you hear the person who called?
Or doesn’t the person who called hear you?
What’s your current version?
Settings - About phone - Build number
(Most current one right now should be 8901.2.A.0111.20200131)