Apparently, when the iPhone 4S (and 5 now) updates to iOS 9 it may alter the Allow Calls From setting – meaning only contacts in the Favorites (or none in iPhone 5) list would be able to call. If you have no Favourites set this would certainly explain why you wouldn’t be getting any calls.
Here’s how to set it right:
Go to Settings on the iPhone
Then Do Not Disturb
Followed by Allow Calls From
Set this to Everyone
Another iOS 9 bug done!