What do prisoners use to call each other?
What do prisoners use to call each other? Cell phones.
This is a pun. The humor is based on the fact that a cellphone is a ubiquitous device used to call people, but it is also composed of the words cell and phone, and even though cell is short for cellular, the room that a prisoner is kept in is known as a cell, so a phone in a cell could be called a cell-phone.