You were perfectly clear and I told you how you could do it. It involves assigning a button to an ICD group and logging that phone into and out of the group. It's a parlor trick, because you would have to log everybody out of ICD group to keep the phones you assigned to ring in the members to not ring. It's not elegant, but neither is your proposed use of the system. This is pretty much the way an end user can control extension appearance on the fly without having full access to system programming.
On my system, I assigned 604 to ring extensions 102-105. At each phone, I set a button to ICD 604, then logged out those 3 phones from all ICD groups (*7360#). Then I logged in one extension to ICD 4 (*7361604). When you dial 604 from anywhere, the button assigned to ICD-604 lights green and the phone rings. However, if that phone is busy, there's no ring at all and the caller gets a busy.
It's a start....
LkEErie