At least you are running iOS 7.
AT&T recently announced they are dropping support for the EDGE network. The earliest generation iPhones and lots of other manufacturers older cell phones can only use the EDGE network. When that goes away the older phones will continue to work, but only if they have a WiFi connection and a VOIP app such as FaceTime or Skype.
I've got an iPhone only because it's one that my daughter got wet and is no longer reliably functional as a phone; I use it only for Internet and as a camera, and although I've known that Skype is an option, I've resisted going down that path.
I'm perfectly happy being one of the "Unreachables"!
But thanks for mentioning it.