To ignore a call and send a text message
To display the Ignore with message option on the Incoming call screen, enable it in the Phone app's Call settings menu.
Tap the Home Button
> the Phone icon 
> the Menu icon 
> Call settings.Tap the Allow ignore with message switch to toggle it On (blue).
Tap Allow ignore with message to manage your text message choices.
To use the Ignore with message option, select it (tap or swipe
its icon) on the Incoming call screen, then tap the Send icon 
next to the text
message you want to send in place of answering the call.
If you tap one of the text messages instead, the Messaging app opens so you can modify the text before sending it to the caller.
This feature can only be used to send a message to another mobile phone. It's not applicable to incoming landline calls.