Using carriers

Hi

Does anyone understand how carriers actually work on the SBC?

I have read the documentation and this states that if the session agent doesn't have the carrier matching the carrier in a local policies policy attributes the local policy will not be used, however we have tested this and this does not work.  The information provided by support suggested that carrier was used as a preference when selecting local polices i.e. if you had two matching policies it would prefer the one that matched the session agents carriers, however we have tested this and it does not work.