While I don't really care about forwarding voice mail, the use of DTMF tones to create a reliable datalink layer is something I was trying to do. A pair of FSR radios, some DTMF chips and some PICs and I have a reliable, low speed (~45bps) data communcations with around a mile range for under $50. This is far more flexible then the model airplane remote controls that cost over $75.

Cisco has some info on it too.

[ This wouldn't have anything to do with advanced remote package delivery systems, would it? -k]

AMIS: DTMF Data Link (kind of) [ZIP]