This is a simple enough process to do, and you're even able to set up your own personalized messages on top of the stock responses. Here's what you need to do to set up your own messages:
- First up head into the main Settings app on the phone
- Tap on "Call"
- Scroll down and tap on "Decline with message"
- You'll see a bunch of pre-loaded messages covering various reasons why you can't talk right now. To edit one of those tap on it and amend the text before hitting "Save"
- To add your own hit the "+" button at the top right, write out your message and hit "Save".