a door bell
A San Francisco Door Bell is when you text someone to let them know you're coming instead of knocking. It's perfect when you're on a steep hill and you don't feel like climbing.
I texted my friend 'A door bell' and she opened the door like I was a hero.
I did a San Francisco Door Bell and my neighbor came out with a coffee.
I sent a San Francisco Door Bell and my dog answered the door.