

It's all easy with a DSP. The Olivetti Quaderno, for example [*great* 1kg
subnotebook/palmtop PC -- sorry for the plug, folks, but Olivetti don't
seem to be doing a good job marketing it themselves :-)] includes sound
digitisation hardware; they provide vocoders for their DSP which produce
varous bit-rates. There's one which gives pretty acceptable voice
quality at 13Kbit/sec, just right for a V.32bis modem.

Their DSP can play and record at the same time, too -- so you wouldn't
need to play "two-way-radio". You can also download code to the DSP
subunit, though you'd need a software development kit for the DSP in 
question [dunno which it is...] if you wanted to produce your own 
vocoder for, say, V.32 speeds.

Paul