I’m doing it like that let’s assume that you have CoS A,B,C
in A you have extensions: 1000, 1001, 1002
in B you have extensions: 1003,1004,1005
in C you have extensions: 1005,1006,1007
Now you want that to extension 1000,1001 and 1002 all others extensions should be able to call so simply in Scenario 3 in “Allowed Calls By” in CoS A configuration you are adding B and C and set “Private” to “Yes”.
Then if you want that to B only C can call you are doing in CoS B configuration in “Allowed Calls By” you are adding C and marking as Private.
And to C can call only extensions from A simply in CoS C configuration in “Allowed Calls By” you are adding A and marking as Private.