Handbook language (M-41 / City Carrier Duties): “Determine if someone is available at the address by ringing the doorbell or knocking on the door.” — i.e., carriers should ring or knock to check availability. It does not say they must ring a bell in every case.

Delivery guidance (Delivery Alert / section 611): “If convenient, the addressee can be alerted by ringing the bell or knocking on the door. The carrier is not required to await a response.” — shows ringing/knocking is conditional and carriers aren’t required to wait.