I do not live in the US, but I'm guessing that AT&T has made the rule so you have to buy their ringtones and not use your own. In the UK the network that does that is Vodafone which you can overcome using a DRM patch on the MP3, unsure about AT&T. I think you should do a bit of searching on the internet, or next person who post may help you.

And the other way is to debrand the phone.