sprint gs3 from straight talk back to sprint

[AC Question]

i have a gs3 that was through sprint and i had it hooked up to straight talk. i went to try and rehook it back through sprint but its coming up as a prepaid phone so they wont hook it up to a framily plan because its coming up as a prepaid phone. im almost positive it wasnt a prepaid phone but am wondering if it would come up as a prepaid because it has most recently been hooked up through a pre paid company. it there any way to take it off of straight talks system so it can be put back on sprint?