Im an hourly ociate. Do i need to be paid for my "on calls"?

I live in the state of Minnesota, I have worked at a retail store for two and a half years. They have me On call two to three times a week. I am only an hourly ociate, do they Have to pay me even if they dont need me the times they had me on call? I have never been paid for it unless i go in. But i am hiring an attorney for harrasment at this same place and poor managment. Do i bring this up to him/her. Im not sure on the law of ON CALLS but ive been told they need to pay me no matter what. Expecially because im only and hourly ociate. Please help me. I am in too deep with these guys and if i get get them for this i would like to, they have screwed me over to much. Thanks