Odyssey,
I know that because I received a letter of invitation early this month with a lot of details. Don't feel nervous, I do not mean that there are two actual interviewes ahead of you instead of one. I have no idea what a phone interview is like but I believe the recruitment weekend is little more than "let us make sure that you are not insane". Anyway, good luck with your interview and UCLA/Harvard application!
Don't worry about that. I believe they will invite you to their recruitment weekend in mid-march after the phone interview, and they won't expect your decision until then simply because they will also not make any final admission or funding decision before they meet you.