What may be happening is that your call is not getting through for some reason.
Make sure your modem is connected properly to the telephone line.
The telephone cable must be plugged into the jack named "Line" if there are more than one slots.
A test you can perform, is to replace the telephone number you are dialing for your ISP with the number of your cell phone or the number of any other telephone line you have access to.
If the call was successful, make sure that the maximum speed (or baud rate) of your serial port is set to 57600 and that you are dialing the correct telephone number in order to connect.