I agree that just walking in is too dangerous.
The guy could open the wrong door, or the wrong guy might accidentally walk in. Cleaning guy walks in, "Hello." "Hi." "Let's fuck."
It makes sense to call on the phone as you get there and say "I'm just down the hall" and let the lady open the door as you walk up. Being on the phone also makes it easier to not talk to any neighbors or lurkers. Phoning also reduces the chance that the last guy or other unwanted visitors are in the room.