When the external service call happens is any menu created on the screen? Does the system which has runtime on it have any change during this external call?
If so, you could interrogate the application that changes from this call. Start an adapter on boot that is set to monitor all and trigger the automation from an event on this change.
Alternatively, you could add a windows for to your automation with a button on it that starts the automation when the external call occurs. (This is if there is no change on the screen when the call is made)
Does that help at all? If you go into more detail I would be able to give you more precise instructions.