cable maybe

You need to make sure you are using the correct cisco cable. I think they send black cables with a DB 25 on it. If you are using a normal rj45 roll cable you need to make sure the adapter on the end is designed to convert a cisco roll cable to the proper db25/db9 pinout.

You could try to test from the router itself to see if the modem will talk to you. I think you telnet to the router address on port 2001 on a 1900 but you need to check the port number. The modem should respond OK when you hit enter.

You also need to check before you dial in that on the show line display there is not a * before the aux port. Sometime it will get confused and think the aux port is in use but it is not. You need to clear the line and see if tha corrects it.

Most the time I have better luck using the console port rather than the aux.