Open Settings
Tap Focus
Tap Do Not Disturb or the active Focus mode
Tap People
Remove any contacts you want to allow calls from
Tap Calls From
Select Everyone, Favorites, or another desired option
Turn off Silence Unknown Callers
Open Settings
Tap Phone
Toggle off Silence Unknown Callers
Check the Ring/Silent switch on the side of the iPhone
Move the switch so it is not showing orange
Increase the ringer volume
Open Settings
Tap Sounds & Haptics
Drag Ringer and Alerts to a higher level
Turn off Silent Mode in Control Center if enabled
Open Control Center
Tap the Focus icon and disable any active Focus mode
Restart the iPhone
Update iOS if the issue continues