What is considered overtime?

I work for a company that says we are paid shift pay so therefore they do not have to pay us overtime. This makes no sense to me how do they pay us shift pay but dont have to pay overtime for anything over 40hrs when they tell us they pay us overtime for anything over 5 shifts, keep in mind we work 24 hrs shifts. im a flight medic and I also work parttime on an Ambulance. Ive been told that they get away with this because of the rail road act. This also doesnt make any sense due to my flying job we dont have a Duty to Act per sey, The company I work for on the Ambulance pays time and a half for anything over 40hrs. however my flight job says they dont have to pay overtime. Please explain to me why? I dont understand their logic. I have friends that work for another flight wervice and Im told they went through the same thing and the company had to pay out lots of retro pay not only to their current employees but to the ones who have moved on to other jobs because of this not being legal