asterisk users - Sep 2004 - Using an analog modem through asterisk (zap channels)

I've tried this before, with no luck.  I've got to try again this
evening, and I'm looking for some help.

Here's my configuration -- pretty simple, really.

Asterisk box -> T100P -> TA750(20FXS/4FXO) -> phones and outgoing lines

I have an analog modem (Ok, it's a TIVO) that I need to be able to
dial out.  Right now, I have the modem connected directly to an
outgoing line that asterisk is also connected to.  As long as the
line's not in use, the modem can pick it up and use it.

What I NEED to be able to do, is connect the modem to one of the
TA750's FXS ports, and dial out.  And it's not the dial plan that's
the problem.  I've gone so far as to set up an FXS port to
automatically Dial() one of the outside lines, which gives the modem
the outside dialtone immediately.

The problem has been that the modem just can't negotiate a connection
with the remote end.

I've tried turning off echo cancelation on the FXS port, and I
probably tried the same thing on the FXO port (which isn't a good
option, since voice calls need to go out over that port, too).

Is there something special I should be doing to make this work?  I've
never seen an answer either way...

Thanks,
Rob