Depending on which version of iOS you're running, you either swipe upward on the phone icon that appears next to the unlock slider and choose Respond With Text, or tap the button labelled Message above the Slide To Answer slider.
By default, you'll get three pre-written options ("Sorry, I can't talk right now", "I'm on my way" and "Can I call you later?"), along with a button that lets you enter text there and then. However, you can customise the prepared messages under Settings > Phone > Respond With Text. You can't have more than three, however: if you want to add a new one, you'll have to sacrifice one of the current options. Tap the one you're willing to lose and type in the new response.
Source : macworld ,rooter15