Configure a remote PBX as part of the extension list
The case was more common than I initially though. The client you are going to move on the MiRTA PBX has several offices and not all of them will be immediately configured on the MiRTA PBX. So you'll have some extensions directly on the tenant and some other behind a remote PBX, like a FreePBX.
Let's make an example with a tenant named "TOWER"
Ext 4401 is on MiRTA PBX
Ext 4402 is on MiRTA PBX
Ext 4701 is on a remote FreePBX
Ext 4702 is on a remote FreePBX
Let's see how you can configure the tenant to allow any extension to call any other extension, as they are all on the MiRTA PBX
You need to start by creating a new extension, like 2000 and use that to register the FreePBX on MiRTA PBX. The FreePBX will be behind the extension 2000-TOWER.
Create a provider not using realtime account and assign the Peer Name as the extension used to register the FreePBX. In this case, 2000-TOWER: