What is my obligation?

I have a web voicemail only account with NetZero (www.privatephone.com) check it out, it's free.

I only have this number posted a few places - my business website, my flickr card.

So whenever I get an email telling me I have a message I know that it's probably not for me.

Today I got new voicemail from a doctor's office calling to reschedule an appointment.

So I wonder, what's my obligation? Do I call the doctor's office back and tell them they have the wrong number or do I just ignore it?