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