Contractors might have specialized skills related to refineries, so you could look for phones that spend small-to-medium amounts of time at refineries belonging

Contractors might have specialized skills related to refineries, so you could look for phones that spend small-to-medium amounts of time at refineries belonging to different companies. You'd probably catch some inspectors and the food truck guy and whatnot, but for the most part it should only be contractors moving around between companies like that.