Oh! I was forgetting another question about the buzzer...
What if someone lives at a building? Then a doorbell could not be used when someone is outside (downstairs), right?
I suppose in that case it would be a buzzer??
Sure, the building may have a buzzer system. As for the cell phone, I think it's highly unlikely that someone would want such an annoying ringtone, so I think in the context you've given, it must be the buzzer (button) a visitor has pressed.