Scam from Facebook. I used an app in concordance with Covet Fashion and when my account was made public for a contest they called me 2 x in the last week.
This is UTMB with appointment reminders. It is an automated system, which is why you get no answer when you call back. It leaves voicemail on mine every time. If your vm greeting is a long one you may not get a message because when vm picks up the recording starts. It's not very long so it may finish before your vm greeting gets to the beep.