Have you tried plugging the RJ-11 cable (4 wire telephone cable) from the modem to an analog jack on the cable telephone interface unit?  You need an Analog (voice) telephone line (or at least an analog connection) with dial tone for the modem (fax) to work. 

 

Do you have regular analog voice telephones that are plugged into whatever interface unit you have?  If so and when you pick up the telephone you get dial tone, then the fax modem should work also. 

 

If you have some special digital only telephones then the fax modem won't work and you would need your regular telephone line back for it to work.  One of the drawbacks with some digital voice over the Internet connections.