It doesn't require Google. It's a service offered by Google that requires the caller to identify themselves. It could be done by anyone, but probably easiest if

It doesn't require Google. It's a service offered by Google that requires the caller to identify themselves. It could be done by anyone, but probably easiest if you have a VoIP line handling the initial call and then redirecting the incoming call to your primary number if the caller is verified (like Google voice). I've only seen this on Google Voice, but I'm sure it's a common enough solution.