Radio Shack Mobile Data Connector
(430-0154)
This device might allow dial up connection over digital phone lines.
It claims to do just that.
I think it might have a voltage line simulator, signal amp, and multiple filters to choose from.
It might be worth a try?
http://support.radioshack.com/support_accessories/60923.htm
http://support.radioshack.com/support_accessories/doc61/61786.pdf