Hi Liam,
As above, the PBX handles all forwarding.
You can tell the PBX what to do via your app. Just tap the status square on the top right and edit the status you want (ie. Do Not Disturb).
Under the Forward To: option, you can pick External Number and enter it.
Now, when you switch your status to DND, it will automatically carry out the forward as you requested. The same method can be used for all statuses.