How do I reply with a message when declining a call?
iOS 6 added the option to decline an incoming call with a message instead of just sending the person to voicemail. To access the three default messages (I'll call you later, I'm on my way, What's up?) just swipe up on your display when you receive a call. You can then select your reply from the three default options. You can also create your own customized responses if your do not like the available choices.
You can also use the "remind me later" option by using the same technique. The "remind me later" option allows you to set a reminder to call the person back at a later time.