Sometimes it’s not the right time for a phone call; and while you could just let calls you don’t want to take go to voicemail, sometimes you want to explain why you’re not picking up. iOS let you quickly respond to a call with a text message.
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.