The AMIS protocol was designed in the late 80s early 90s to allow Voice mail systems of different vendors to automatically transmit voice mail messages to each other.

It uses DTMF tones to communicate back and forth, establishing who the voice message is for, where it is coming from, and for the destination system to specify if the mailbox is full, to provide a forwarding number, etc. The use of timeouts, checksums and acknowledgements make this a reliable protocol, with a variable payload length. The analog voice message itself is not digitized, and is played real time. A sample session looks like:

-A dials B
-Using DTMF tones, A and B agree on a protocol.
-A tells B for the message is for, who it is from
-B confirms information is correct and mailbox is available
-A plays voice message "Hi Billy, this is Jill..."
-A signals message is over
-System gracefully terminate.

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.

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