Is this an internal or external modem?
If internal - then it is most likely some form of
WINModem - which is to say, a modem designed by the vendor to be used only with Windows.
Fortunately, many of these modems have been reverse engineered to work with gnuLinux. To use them, you need to identify the
chipset of the modem - the manufacturer designations are useless.
To identify the chipset, you need a script called "scanModem" available from the
linmodem website.
Run the script (the link has instructions) and post the content of the ModemData.txt file it produces.