How do I import an mp3 that was sent as a text to my iphone 4 as a new ringtone?

-The phone is from Verizon and is not jailbroken. We found a website (www.pisamba.com) that will send a ringtone to your phone as a text message, it comes in to my phone as a quicktime song. Now how do I add that as a ringtone to someones contact?