I think though we've missed the whole point of the question, he does not
want a VoIP solution as much as a callback system and I think Google Voice
does that. You go to their webpage and schedule a call, and their system
calls your registered phone line.

When you answer, it calls the number you requested and connects the two.

Right... but I interpreted his question to mean "Can I make a Google
Voice call, and run a dialup modem connection over it?" My belief is
that Google Voice uses a VoIP backbone to carry the call; the
endpoint connections are over the PSTN but I believe that the
long-distance part of the call is via VoIP.

Of course, you need two phone lines, one to dial up with and one to receive
the call.

Probably so. You might be able to click the "Make a call" button,
immediately hang up, and receive the first leg of the call on the same
phone line, but the timing would be very tricky.