all in one fax hook up

i have a hp7280 all in one printer. i have accomplished hooking everything up but the fax. my telephone service is with a cable company so i do not have a wall outlet to hook the fax directly to. i have the fax telephone line inserted in line 1 and when i print a fax text it keeps telling me i am not hooked to line 1?? i have the fax and my telephone hooked into my modem with a splitter - the phone works fine but i am having difficulties with the fax - please hepl me if you can.

thanx,

bob