COS not working as expected

From previous comments in these forums and the COS documentation, when an extensions is in multiple COS’es any allows should be cumulative.

However this is not working for us. I have a feature code *280 that is disallowed in all COS except one. If I then apply this COS to an extension that is in another COS (ie orginal COS allows international dialling), the effect is not cumulative and the extension still is not allowed to dial *280.
If I add *280 to the main COS then it can be dialled by this extension (and everyone else, which is not wanted!)

Am I doing something wrong, or do I need somehow to order the COS’es