Getting into the subject, if you have already configured fax modem to your PC and if you are able to send the faxes successfully through windows fax scan and unable to receive them, then one thing you need to quickly check is the Fax Settings.
Click On Tools--Fax Settings in Windows 7 or In Windows Vista, in the fax setting dialog check whether you allowed you modem to receive fax calls. If "Allow The Device To Receive Faxes" does not have the check mark then you will not receive the faxes though you can send faxes. Check Mark the option and click on apply and Ok.
This is out of my experience where i spent more than half hour trying to figure out where exactly the issue was, after replacing different kinds of modems. You can also check my earlier post about how to use Windows Fax Scan as any file TIFF converter.
If you are using windows xp and would like to use your computer to send, receive and print faxes then you can have a look at the good tutorial posted here To configure the fax service in Windows XP,
You can also use the General Help Form To Get More Help On Windows Fax Scan