Another cool thing is that most elevators have a phone line. It's used for the emergency button but usually also accepts inbound commands. In our old office building the installers had written the number in pencil and I called it a few times. It answered. Unfortunately it didn't activate the speaker, there must be some code. Because this would have been amazing to play practical jokes on my friends.

I tried some basic DTMF but couldn't figure it out, sadly..