I've been prototyping a dail-up concept (like CSD) over 2G networks. Combined with heavy server-side compressing, the goal is to achieve speeds faster than GPRS in areas that have no data coverage to speak of. TCP over sound was the first part. I'll have to study this implementation.
Note that your audio will get fed through the AMR codec and then limited by the bandwidth available to that. For best results reverse-engineer the codec so your symbols are maximally distinguished by it.
It should not be possible to exceed the carrier's CSD rate over the link, but it's an interesting way of circumventing their billing.
