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.

It is probably hooked up to an alarm panel. It is very common for those
devices to use the same kind of wiring for a telephone. As far as the
other 2 screws not being used, I think you can wire some heat sensores
as 2 wire or 4 wire, depending on what kind of application is its used
for.