a doorbell is not required in a single family home under the nec. however if you dig deep enough in the national fire protection assn book you will find something to the effect of "dwelling must have at least one egress capable of exterior emergency notification." (not an exact quote) thus the fire protection people want arriving emergency crews to have some means of alerting the people inside from the outside. thus as i have been told a door knocker or a chime meets this critera.
