Could you achieve a more high throughput version over voice? I wonder, after compression what the data rate ends up being. I'm reading gzip can get 2:1-4:1 with text. So about a kb/s?
Yeap, except voice is now carried over a digital channel, so it'd be digital-over-voice-over-digital :)
Over a cell phone connection you will get much less. GSM data modems gets to bypass the voice codec, and are still limited to 9600 bps. An acoustic modem over a GSM connection will get less than that, if it will connect at all: A full data rate GSM code is compressing the audio down to about 12.2kbps using codecs geared towards reproducing voice as well as possible, which is going to be brutal to a regular modem and far more wasteful than GSM data.
I'm guesstimating you'll find an upper bound, assuming "friendly" telcos that don't start rate limiting, or dropping SMSs, at no more than about 6kbps. Quite likely less.
GSM data (which is normally possible anywhere where you can get a GSM voice connection) is 9600bps and uses the full, raw GSM data/voice channel. You'd not be able to exceed the channel bandwidth. Then you have the SMS overhead. And you're unlikely to manage to max out the channel.
The issue is that it quickly ends up costing about the same amount as a data connection would in the first place.
Unfortunately, voice calling uses lossy compression, so your data rate would be somewhat limited as you struggle to be 'heard' over that.
That by itself could get you up to about 0.5kbps for text with decent compression, with no other effort.
I think we lost that feature in the transition to 3G.