This alarm doesn't do much, but it does just what I needed. We have a second home which is unoccupied much of the year, but which we leave heated to 55 deg. Here in New England the temperature can get down pretty low. The town actually requires us to keep the water on at least a trickle all year, because the water lines can end up below the freeze line. All I needed to do was plug the unit into an open phone jack, key in my cell number and hit the test button. My cell rang a couple of seconds later an I got a recorded message saying there was a temperature problem at the house. There is a battery backup in case the power goes out so the FreezeAlarm can still call out.
Other units, which cost quite a bit more, will call to more than one number and even turn up the heat before you arrive. This seemed like overkill to me so we got the basic unit. It keeps calling the programed number every two hours (though there is an option to do it more frequently) until the phone is answered. They give you an option to "really" test the unit, by placing it in a refrigerator. Doing that might expose the unit to damaging condensation, so I didn't try.