This is a test of commonsense. Complete the next sentence:

How to respond to incoming calls with a message on an iphone
Receive a call on your iphone.
You'll see the option to respond with a text message when you get an incoming call.
Tap the message button on the incoming call screen.
This looks like a message bubble, and can be found on the right side of the screen, above the slide to answer or accept button. Tap one of the preconfigured messages.