Had that happen in an elevator. Elevator call boxes have a phone line for emergencies.

RING!
(pushes button)
“Hello?”
“Hello, who is this?”
“This is an elevator.”
“No, seriously, who is this?”
“No, seriously, this is an elevator.”
(Everyone in the elevator starts laughing)
“OMG… this really is an elevator!”
“Well, yeah, that’s what I told you.”
“How are you talking to me?”
“The call box started ringing so I pushed the button.”
“Um, OK, can you hang up now?”
“Well, there’s no real phone, just a button and a speaker, so I think you have to hang up.”
(CLICK)