Lifted from Admin Guide:
If an external gateway does not require authentication, its access number can be stored in one of the 99 speed dial slots to allow ad hoc direct dialled gateway calls. To do this, the user dials the gateway's speed dial, followed by a *, followed by the target number. That is <gateway-speeddial> * <target-number>. For example, the gateway access number pp(ob200333456) is stored at speed dial 8, and the user can dial 8*14085551234 to call 14085551234 using the given gateway.
Note: At the present time, only gateways that are accessed with an OBi number can be used this way.
This seems to be by far the easiest way to make an outgoing call using another OBi located far away. Dead simple.