Motorola V710

I have an mp3 ringtone that I downloaded from Sony music box which is one of the programs available through Get It Now on Verizon. My phone will play the mp3 from the audio file menu. However, when I assign the mp3 as the ringtone on my phone, it does not work. When my phone rings, I get only the generic ring. What must I do to make the mp3 serve as a ringtone? I have done nothing to my phone and have added nothing since I bought it. I appreciate any detailed instructions.
Thanks.