OK here's what I got on a 120 with f19 SW
0, 1, 2, 6, 7, 8, 9, work fine
3, 4 & 5 do not ring ans. machine (it won't answer)
0-9 all ring a single line set.
7,8,9,0,1, all have what sounds to me like a normal ring pattern. 2,3,6, have a double ring pattern.
These tests were done using inside calls from a digital ext. to ext 152 with an ans. mach in 152 and a phone in the ans. machine.
I would expect the same from a dx80
Mark