Look at the charging port with a magnifying glass - you'll probably see dirt in there. VERY carefully, with something non-metallic, clean it out. (Check the plug on the cord too.)
Well, if it's not giving you any other issue than saying it's charging when it's not, and, it doesn't interfere with charging when it's supposed to be, I don't know that I'd really worry about it. You can contact your carrier, as that sounds like hardware, and see what they have to say, but, I wouldn't lose sleep over it.
This can be a major problem. My phone has been doing this for a couple of weeks and now it is sometimes difficult to switch on - it only shows the battery charging meter when I try to boot it. I'm wondering if this problem started after I used a non-standard battery pack to charge the phone...