I had this working at one time but it was hit or miss and then not all fax mahines support this very well. I finally went with an outside source that gets you a local or 800 number and faxes come to you by email or you can login and pull down the stored faxes..cost anywhere from 5 to 40 a month its according to the service provider you get..
I used faxmicro.com and got an 800 # 10$ a month flat fee no per page or per minute charges (local number was only 5$ a month). NICE thing is NO BUSY signel so your can receive a lot of faxes at the same time.
Hope this helps.