Data conversion problems.

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First thing you should do is see which voice samples your modem supports. 2-bit
voice is unacceptable. 4-bit is tolerable, and its better with compression. 8
bit is obviously the best. Most modems have an AT command that will show
you the bit rates available. I don't have a link to Rockwell chips, but the
USR Sportster info is at http://ww.pcd.usr.com/techref/techref.htm. For those
modems, you issue an AT#VCI?, and you'll get something back like (copied from
their example):

USRobotics;GSM-B;8
USRobotics;G.721 ADPCM;4
USRobotics;IMA ADPCM;4

Also, make sure that you're using an approximately 8KHz method, as that is
what [most] phone calls will use (the phone company can play tricks to conserve
THEIR bandwidth by dropping the call to 6KHz-7KHz, sometimes a bit less). 
Again, with Sportsters, the AT#VSM? command will let you know whats available
for compression/bit rates. Again, using their example, the output might look
like:

128,GSM/USR,8000
129,G.721 ADPCM/USR,8000
130,IMA ADPCM/USR,8000

In the above example, they all do 8000Hz, which is good :) So, basically, you
can use compression method 128 to get uncompressed 8 bit samples at 8000Hz,
or you can use method 129 to get 4-bit compressed G.721 samples. 

But, again, 2 bit samples won't do you much good, unless you have an incredible
codec, which I'm sure the phone company would love to pay lots of money for...
After all, "good" voice quality at two bits? Hell, I'd buy it :)

	-Brian