Turns out, I didn't even know you could answer a call with a text before.

I assume this was added in PR1.2 maybe?

Well, nonetheless. To edit the text, when you receive the call, and select to send SMS, you can edit it right on there.

It's just a standard message, so you can type right on.

If you mean change the default to a different one permanently (to not have to manually edit it each time... then I'm not so sure)