I went down this road when the SBX first came out but you can now use any station for a fax. There are several places you must and change programming.
Station ATT 2 to designate the fax position and Station ATT 3 make the warm line "hot". Also in Sys Base program Sys ATT you select the CO for fax detection. There are other settings but these should get you going. I run an SBX in my shop and have my fax running as ext-107. My third line is my fax. Detection works pretty good!