Except that many contracts now have 'unlimited' voice minutes.

Unfortunately, voice calling uses lossy compression, so your data rate would be somewhat limited as you struggle to be 'heard' over that.

Before this fancy GPRS thing, we used to have something called CSD (circuit-switched data), where your phone could use a raw GSM voice circuit to send data instead of digital audio.

I think we lost that feature in the transition to 3G.

GSM data still only gave you 9600bps, though.

The later versions let you bundle multiple channels (at n times the cost) to reach 57 kbps http://en.wikipedia.org/wiki/High-Speed_Circuit-Switched_Dat...

Has anyone tried transmitting a QPSK-125 signal over a cell phone call? I wouldn't be surprised if it worked.

That by itself could get you up to about 0.5kbps for text with decent compression, with no other effort.

that's what error correcting codes are for :) If you design things to play nicely with the compression algorithm that's used by trying to stay within normal human vocal range and stuff, might be okay.

True, but if you were somewhere where data was unavailable or you didn't want to use a local data connection, it might be a nice way to force a map app to update to your current location, get an email, send your location to somebody, or answer a chat message.