I assume you have already checked the obvious i.e.
1. Is there something else using the phone line (if it is shared).
2. Are the correct modem drivers installed.
3. Tried ringing another number with the windows phone dialer(home/mobile) to see if the modem is actually dialing correctly.
Has the modem been working ok until now or has it just been installed????
It would be helpful to know more about your setup (i.e. os, make/model of the modem involved,etc).
Hope this helps