asterisk users - May 2006 - Sharing an outside line between a modem and a TDM400

I have an analog modem and a TDM31B and one outside line.
I want to use the modem to dial out on demand for a PPP network connection
and also use the outside line with the TDM31B. I am willing to have the
modem work at 28.8Kbps. I want a lock of some sort so that voice calls do
not break in on data calls and vice versa.

My first thought was to have the modem connected to the TDM31B and have
asterisk avoid contention. However I could not get the modem to connect to
my ISPs modem. I tried forcing the modem to use slower speeds, but it would
still not successfully negotiate a connection with the ISPs modem. I tried
this with the echo canceling features on and off.

My second thought was to have asterisk check a lock file when using the
outside line's zap channel. However when using PPP in demand mode the
tty for the modem is continuously locked so I can't simply use the tty
lock file to control access.

My third thought was there might be some software that allowed the TDM31B
to function as modem. While I did find stuff software relating to faxes,
I didn't find anything that would make it function as a computer modem.

Does anyone have any suggestions on how I might be able to do this?