Ok, now I see your point.

However, I think that you can't negotiate a 56kbps link ( http://en.wikipedia.org/wiki/V.90_%28recommendation%29 ) over a cell phone.

Ordinary copper telephone service has enough bandwidth to transmit a full range of audio signals, in fact more than the voice can produce or the ear can hear. Modems exploit this fact.

But cell phones employ lossy compression, and if I understand correctly, one of the "first things to go" are the higher frequency noises. Why transmit what the ear can't hear?

Oh, somebody smarter than me has addressed this question: http://superuser.com/questions/748154/use-a-smartphone-as-a-...

Ah yes, I had forgotten about that. I seem to recall that older ones had modem functionality, I certainly recall having a fax feature at some point.