After you have logged into your babyTEL web portal, head to Service - Service Continuity
Enter the destination number in the "Forward to number" field, click enable, click Save.
You can also setup Service Continuity for the other numbers in your account if they are independently routed.
Remember that there is a distinction between this feature and applying a Call-Forward at all times in your account. If you setup a call forward at all times, inbound calls to ANY of your numbers will be forwarded to your destination number whether your PBX is registered or not. The service continuity only happens in the event of your system not being registered, and has the possibility of multiple forward destinations configured.