Use Local-only directory numbers for 0 in each cluster element. From the Help file:
Local-only Directory Numbers - Description
Within a cluster, you can program directory numbers that only exist in the local databases of the member elements. These local-only directory numbers allow you to assign the same extension to a service or department that is local to the site supported by a cluster element. For example, you could assign local-only extension number 1000 to the security department at each site.
A local-only directory number is not listed in the Remote Directory Number forms of the cluster elements and is therefore not cluster dialable. A user on the same element can call a local-only directory number by just dialing its extension number. However, a user on a remote element cannot. To call a local-only directory number, a user on a remote element must dial the CEID digits followed by the local-only extension digits.
