How can I customize a response to use when I am unable to take a call?

[AC Question]

I am unhappy with the selection of messages you can send when you are unable to answer a call. I thought that I had added a "custom" response that wouldn't be so harsh, but it has not appeared among my choices. However, I now have a yellow banner going across the top of my messages screen that lists my conversations that says, "Auto Reply is ON".

 

[hallux]

On my Nexus 6P, using the stock dialer. I open the stock dialer, tap the overflow menu (3 dots in the right end of the search box), select settings then quick responses. I can edit the responses available when I choose to reject a call with a message in response.