There are chips which perform the voice compression/expansion.  They can't
be expensive, because they exist in many phones connected to PBXs or on the
PBX line cards, as well as in a lot of equipment which compresses
voice-grade circuits to save the cost of long-distance, leased T1s or
satellite circuits.

I can't remember the generic term for these chips.  My impression is that
this was a big deal 10 years ago, but circuits have gotten so cheap that
it isn't done much now.

Lew
