Why is there a cell phone signal in the elevator?

Because there is something called: cell phone signal amplifier.

Mobile phone signal intensifier (also known as mobile phone signal amplifier and micro repeater) belongs to mobile phone signal relay equipment. Working mode: outdoor signal is wired in, and then transmitted indoors after downlink amplification to make up for the blind area of mobile phone base station; Then the cell phone signal is transmitted to the base station through uplink amplification to realize the call.

It is usually installed in elevator shafts, underground garages and other places.