Telephone wiring and heat detector ???

Hi. I'm am currently living in a house that I do not own. In the kitchen, on one wall fairly close to the ceiling, there is a small round device that is labeled "heat detector." Today I looked inside the heat detector and noticed that instead of operating on a battery or being hard-wired to the electricity, it is wired to the telephone line. Does this make sense? It is an older-type telephone line consisting of red, green, yellow and black wires; however, instead of each color wire being connected to one of four different terminals, as would be the case on a phone jack, the red and yellow wires were wound around each other and were connected to one terminal, and the black and green wires were wound around each other and connected to a second screw/terminal. The other two screws were empty (no more wires to attach to them). Can someone explain this to me? Thank you.