You cannot use the AUX port available on the ASA in order to connect to the modem. But, you can use console port to connect the modem.
Complete these steps in order to use the console port to connect the modem:
If a PC is used to configure the modem, use hyper terminal with the speed set for 9600 baud.
Note: Use the standard Cisco Rolled Cable along with the modem adapter.
The modem settings work for US robotic.
Refer to ASA: Attach a US Robotics Modem to the Console Port for more information.