This is a design flaw, pure & simple. If you haven't experienced it, yet, there's a good chance you will.
The charging port is soldered to the circuit board. It is not encased in plastic and affixed like it should be. After enough pushing and pulling the charging/mini USB jack in and out the port loosens up and eventually can become disconnected totally.
I bought a 1st day phone with no insurance. I really don't think Radio Shack mentioned it but I could be wrong. There was near bedlam in the store so maybe he did and I don't remember. I was too excited about the phone to consider it.
Anyway, two different Sprint repair stores I visited in person said I would have to buy a new phone as they would NOT open the phone up.
I said to hell with that. Found a brand new one, in the box, on CL for $350. Almost bought it.
Then found a non-Sprint repair shop that would work on it. She (the tech was a female and kinda sexy, btw) said she sees these a lot as "it's a poor design". She said the phone was too new so she couldn't get a replacement part but the port looked OK so she would try and re-attach it. She soldered it back and it works fine and, supposedly, is stronger than it was originally.
After stressing out for a couple of days I ended up spending $47, although I did purchase an outside-the-phone battery charger and an extra high capacity battery from Amazon so I won't have to use that port much, just for data transfer.