Avoid WinModems like the plague, because these modems are designed for Windows O/S only.   They're cheaper than regular modems because they're lazy ( or smart depending on how you look at it ) and depend on the Windows O/S to do much of their work for them.   Linux drivers are only now beginning to appear for such modem.

echo "atdt5555309" > /dev/ttyS0 

   honestly, it doesn't matter what you put between the quote marks in the preceding commands.   It just has to be some text --- any text

   If your hear the modem pick up and dial, you've won the game of hide-and-go-seek and know what device the modem is connected to.

echo "atv" > /dev/ttyS0 

   If you hear the modem pickup and dial then, it's connected and had dialed a number

ttyS1, ttyS2, ttyS3 

   If you can't hear anything, then try using the other serial ports by substituting the above port to the echo command

echo "atz" > /dev/ttyS0 

   After your modem is found, send the following command to the modem to kill the connection