If I say that 8709 works like it should, is that true? I think even though the buzzer rings the same phones, its technically two different groups.
When you dial 8810, it gives you the out of service message, correct?
I think what you need to do is create either a ring group or hunt group, with dn 8810, and put 8809 in it for members. Or because 8809 is already multicall keys, you could change the Speed Dial of 0 to point to 8809, while adding a second speed dial of 8810 to point to 8809.