Small Business Server Fax issue

I have SBS 2003 and the fax service does not seem to be working correctly,

When system 1st installed the fax service worked fine with a US Robotics External modem, then it just stopped working, I have changed the Modem over to 3 different external modems with up to date drivers, however this still does not work,

What happens is the modem will pick up the call however the server will not answer the fax call coming in.

Is there a fix for this or has any other person had the same type of problem.