Anything can happen with a device designed and manufactured by humans. However, I would start with a visit to the phone that gives you 3G service and investigate their coverage and bandwidth data transmission.
It is possible that the provider (telephone) will not supports 3G (not for you) that your transmission is mediocre and then the blame will fall to your iPhone 5, which goes to the other (iOS 6.1.4) operating system for smartphones tablets and most advanced in the world. Everyone, at some point, this has happened to us ... believe me