Callaway was added to the roster back in April as an undrafted free agent. He was a local guy, having played his college football at Sagniaw Valley State. His chances of making the roster weren't good, especially since he plays at a position with pretty good depth, so it's not surprising he was let go.
Pratt was actually on the Lions' roster up until the time when Dre Bly was signed. Bly's arrival meant the end of the road for Pratt, but now he is back, likely because the Lions are hurting for depth in the secondary right now due to a string of injuries.