In article ,
wrote:
Can I use Google voice with dialup?

Trying to do any sort of modem communication, over any sort of
voice-over-IP audio pathway is usually a horrible exercise in
frustration. VoIP tends to be vulnerable to packet loss and timing
jitter, and many VoIP connections use voice-grade lossy codecs. Even
fax connections at 1200 bits/second are difficult to get working
well.

High-speed modem connections (e.g. V.90) require a much cleaner and
more stable signal pathway... "dropouts" and timing jitter and
distortion will bollix them pretty badly. You really need a
circuit-switched connection.

I have not personally tried doing dialup-modem connections over GV,
but I'd be somewhat surprised if you found it to work acceptably.