How can I disable my home's ADT monitor?

I just bought this house, and discovered there was an ADT home security monitor in it. As it was “dead”, I thought nothing of it (except that I’d have to get it removed one day), but during a recent power surge it started to go off.

Now it beeps every 5–10 minutes, and informs me that it cannot detect a phone line. This is unsurprising, as I keep a cell phone, but I can’t get it to stop beeping! How do I turn this thing off?!