Overtime and Callback Pay

Hi- I work for a hospital, and have approached HR as well as administration regarding this matter. I have noticed that if I sign up for a shift and receive call back pay, I do not receive it should I enter overtime. Any other facility I have worked at, I have received my callback pay- up until I reach overtime hours. In other words, I cannot 'double dip' for the same hour, but my callback pay was never taken away to cover my overtime pay. I have been informed that going to the labor board will inevitably result in my termination. I have researched this for over a year and a half at this point, and have come to a pass where I am tired of working and not being paid appropriately. Can you advise me further on this? Thank you