How to set a textone as a ringtone for the iPhone 4S?

I heard a ringtone the other day that I thought was pretty cool. It was a continuous tritone. It basically sounded like someone was receiving text messages non stop. I really want to know how to do this but I can't figure out a way. How do I do it? All answers appreciated.

Settings than sounds

Your iphone can use ringtones as text tones. It is a feature of the iphone 4s.