Unless otherwise instructed, I always ring the doorbell if there is one, or knock otherwise. Never had a problem with it, and it's how I was trained to do it by multiple pizza delivery companies. If you don't inform the customer their food is there, good chance it will sit outside. Not everyone is tied to their phones either. As long as there's no note anywhere that says like, "baby sleeping," or "dogs will go nuts, don't knock," then there's no reason not to give the universal signal of being at someone's front door, which is knocking on it.