Hi Sophie,
If you edit your cluster, there is an input box called Clusters to include. In that box you can type the names of other clusters that you want to be able to call from this cluster. So let's say you have created cluster_a cluster_b and default then you can edit cluster_a and type...
cluster_b default
This will allow phones in cluster_a to call any phone in cluster_b and any phone in default.
Hope this helps.
Kind Regards
S