   How fast do the fastest modems go?

V.FAST is targeted for about 28kbps; 14.4kbps is ~$500, and 19.2 may
be here soon too.  (Remember to ignore compression figures which make
your modem go faster on compressible data, because voice gets
compressed better than that, and encryption makes further compression impossible.)

   How far can voice be compressed?

The STU-III phones on the market from AT&T, Motorola, and (I forget who else),
give you a choice of 9600, 4800, and 2400 baud.  The 9600 and 4800 are CELP,
and I think the 2400 is some earlier military vocoder stuff.
9600 is decent, 2400 is pretty artificial, anything less is Speak&Spell.
16000 can produce quite adequate sound.  There's also some 8 kbps CELP around.
DSPs have made it possible to do all this in real time for cheap.

Old-hand government crypto have trouble adjusting to systems faster
than 9600 baud, because if the voice on the other end sounds good,
you assume the crypto hasn't been turned on yet :-)