I looked for this driver as MDP 3900 on both Dell's and Aztech's download page.
I finally found it when searching Dell's message boards. A tech support person mentioned the this link
in response to a request
The modem drivers required for...
-MDP 3900 PCI Modem Emulator
-MDP 3900V-U Modem
for Windows 2000 Professional
The following files are found inside the driver download file.