defjon,
You probably just need to change your modem init string, and maximum connection speed.
For the modem init string, try going into ASA, under Connection Properties go to Edit Devices and find the data module in the list that you intend to use. Double click the entry and copy the modem init string into your windows clipboard.
Once you have it go back into the Call Flow Extract System Configuration - Configure System and paste your string in thhe Modem Initialization section. Make sure you change your Connection speed to 9600.
We are using a ACCURA 28.8 modem, which has a monster init string 'AT&FE1V1X4Q0&C1&D2S7=255 S0=0'. After following these steps, everything worked cool for us.
Barry