Earthing of a communicator is a good idea cos of the points alrready noted. Mainly this is to offer some protection to the device to be able to ground any spikes etc that do enter the communicator via the huge external aerial that is the BT network. If you dont and you get a strike anywhere near you, then it will go through your dialler then down the little alarm cable, under your boards etc and find some way or other to get to ground.... might be through a pir near a pipe? might be you as your using the keypad? who knows.
But if you earth it at the communicator then it follow the path of least resistance that you know is now the mains cable feeding it to earth.