Can't delete default ringtones :(
I can't delete default ringtone on my iphone5 even on my ipad mini. i purchased it by itunes
iPhone 5, iOS 6.1.3
In the iPhone Settings icon you'll find a way to tell it what ringtone to use. So you can tell your phone to use another one without actually deleting it.
You delete it in iTunes. You'll find it shows up in a separate pane as 'Ringtones'.