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).