In iOS 6, you can now silence a call and quickly reply with an iMessage or SMS to let the caller know what’s up. You can also set a reminder to call the person back. Simply slide up on the phone icon during an incoming call to access the reply and reminder options.
Apple added a few basic default messages like “I’ll call you later” and “What’s up”, but you can add your own custom replies. Go to Settings > Phone > Reply with Message, and there are three message slots you can edit. However, choose carefully—whether you keep some of the default messages, you only get three slots total.