I would only respond as "busy now get back with you later" then never do so.
I actually disagree with this one. I think it's imperative, in all things, to DO WHAT YOU SAY YOU ARE GOING TO DO. If you're not sure you're going to call her back later, then say so: "I'll try to give you a call back tonight, but I've got a busy evening planned -- no promises. If not, we'll definitely catch up tomorrow" -- or some such.
You want to be the one person acting with total character and integrity right now.
(Actually, that's not a bad way to live always).
"What is best for my kids is best for me" Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712