I was about to say this.

If they can’t give me a callback number that is publicly listed on their web site, then they’re most likely a scammer.

With Google, however, this is a scarier proposition than with most companies. If someone from my phone company, or my bank, or my insurance company called me, I could very easily call the actual company and talk to a human to confirm. I have no idea how I could ever talk to a human at Google. I’m not sure they even have a public phone line.