I suppose it might be possible to make a UI rule in the work order module that presents and error message to the user if a work order is related to a service call that is in the "new" status.
People could still make work orders but could not relate them to Service Calls in the new status, althought a clever user could still get around this rule by saving the Service Call into something other than a new status, creating a work order, and then changing the status back to New.
All is correct. When I have created the work order I can change the status of the related call. When the service call has the status "New" I want to block the tab "workorders" or the buttons on this tab.