Some older versions of sets appear to lose their BLF capability when connected to Ver. 6.X software. While the internal auto-dial function of buttons remain, a BLF attached to a M7310 set will no longer function. Likewise, an internal auto-dial key on the set (M7208, M7310) will still call the appropriate extension, but will no longer give an indicator that the set is in use. A real quick test for version of the set is to use contrast adjustment [F][*][7]. If you are only able to adjust to 4 levels the set is of an older vintage.