When you create the Tenant, go to Calls Routing and add in Allow Tenant Trunk all the Tenant you want to communicate with.
Then you go to the previously created Tenant and you are going to create a Trunk and in the type you select Tenant and in the Tenant* field you associate it with the Tenant you want to communicate with.
Finally you go to Outbound Routes Create a route, select the trunk created previously and define in the Dial Patterns the prefixes or the patterns of the extensions you want to call.