Sprint Nexus S 4G ringBACK tone

[Kattitude]

Can someone PLEASE tell me how I can get the mp3 ringtone I have downloaded on my Sprint Nexus S 4g to be a RING BACK tone. I would like some of people that call me to be pleasantly or unpleasantly surprised and would like to be able to change it according to my mood.

PLEASE HELP!!! I have spent 2 weeks of my life trying to get customer (ha!) service of all companies that MIGHT be connected to my phone or issue.

Again, Please Help!

Thank you!

Kat (on her ninth customer service life,( none left)