Ringbacks with UScellular?

With Verizon Wireless, at least, if you want Ringback Tones, you need to log onto your cellular account through your computer and into the Ringtone/Ringback tone store that way. There's no way to do it directly from an iPhone. Check with your carrier for specifics.

To my knowledge, ringbacks have always been a carrier feature and are generated and stored on the carrier side, since it is part of the calling feature, which of course is handled by the carrier.