das250:
I was also investigating the use of warning module arduino that can call another phone without it being rung up and using the ring tone and length of call to issue basic information to another module that listens for ringing and length of ringing .
I though I had replied to this a while back but it never seemed to make it to the board.
You could maybe use DTMF as it allows 16 different tones so with a combination of *, #, A, B, C, D & 0-9 you could maybe send some quite complex messages.