Are you sure about the D-Link?
I didn't see anything in the documentation about the unit being able to handle a dial-up modem. I got the impression on my DI-713P that the serial port was just for direct connection to the unit if the network config got fscked.
~~~)-Steven----
"I want you to remember that no bastard ever won a war by dying for his country.
He won it by making the other poor dumb bastard die for his country..."
General George S. Patton