First, you will want to make sure the correct straight through cable is being used to connect the modems.
Next, you will want to confirm you can communicate with the modem from the Linux host. We provide a utility similar to cu (from the uucp package) that will allow you to connect:
# dinc 9600 /dev/ttyx##
Where ttyx## represents the actual tty device. Once connected, you should be able to issue AT commands to the modem.
Press ~. (tilde, period) from a newline to exit dinc.