Ringer
My phone does not ring, when someone calls except when connected to my car.
iPhone 4s, iOS 9
Question marked as
I'm not sure how or whether the switch affects the ring when connect to a car but you may want to check the ring/silent switch on the left side of the phone. See: https://support.apple.com/en-us/HT208353
If that doesn't help, also try restarting: https://support.apple.com/en-us/HT201559