I received a new iPhone with a new number so I wanted to cancel my previous iPhone and number. I wanted to remove this number from my Apple ID so that it was removed from being used in iMessage and Facetime on all of my devices.
UPDATE : NOVEMBER 18th, 2014
Apple has released a web-based tool to help de-register your phone number from your Apple ID/iMessage/Facetime which replaces a lot of the “older” methods found on the web.
The web-based tool can be accessed by going to: https://selfsolve.apple.com/deregister-imessage
The nice part about this web-based tool is that you can still de-register, even if you don’t have your iPhone any longer.
Hope it helps!