I never did a phone interview with my POI, but I had been in contact with him over email since last summer. He emailed me and invited me to come to the open house a couple weeks ago. But, since I haven't received an 'official' invitation yet, I think that maybe not all of the invites have been sent out? Some of the faculty might still be deciding who to invite - by your user name I'm guessing you're in clinical - I'm in social psych so there might be a difference there too. Good luck!