I have a Samsung a117 pay as you go phone.
In the manual Q&A here is what it says:
Q: Why can't I download ringtones from a non at&t site?
A: AT&T service does not support downloadable ringtones from external sites.
I don't like cingulars selection of ringtones so that is why i'm asking.

pay as you go might restrict you in some way. im not sure.
i have cingular phone (not pre pay though), and I create and download all of my ringtones from this site.

http://www.ventones.com

its the best. never been charged.