I called  and Google does actually call to see if your number is legit. I take it everyone on here possibly has their own business or their phone number is a business phone. If not, someone may have submitted your number as a business number either on purpose or most likely accidentally, and therefore that is why they call and check.

It's Google making sure your adwords calls are going to the right #

This is a legitimate call from Google. They verify your number when you advertise on Google and include a phone number in your advertisement.