The phone app in iOS 6 has a number of updates, one of the most useful is the ability to reject a call and have your iPhone automatically reply with a text/iMessage to the caller.
iOS 6 comes with some defaults like “What’s Up?” but you can easily customise these messages.
Visit the Settings app, then scroll down and go into the option for Phone, you’ll see the screen shot below.
Select the Reply With Text and the rest is pretty straight forward. Choose which of the default messages you would like to customise and type in your message.