I recently bought from a friend a slightly used iPhone 4S for my wife that was already loaded with iOS 5.1 and it was stock. He would comment me that from time to time the calls would drop in certain places with the iPhone but a friend of his using a different phone in the same area would not. She recently told me that she too started experiencing sudden call drops as well. I have a 3GS JB and use it all over with no problem. My theory as of right now is because it's using HSPA+ 4G 14.4 at all times whenever possible, but when it's not in a 4G capable zone, it drops trying to rollback to 3G or something. Has anyone had this issue before or something similar. I have her currently on h2o which is an MNVO of AT&T.