With Android 4.0 (Ice Cream Sandwich) or any newer version, you can decline a phone call while sending the caller a custom text message. This is not a new feature, but the latest OS has different settings. To customize auto-response texts, go to Phone > Settings > Quick Responses. Tap one of the pre-written responses to change it. When your phone rings, you would normally slide the circle to the right to answer the call or to the left to send the caller to voice mail. To send a response text, slide the circle upward to the message icon and let go. From there, you can select a pre-written message or write a new one.
Source :- informationweek