You can do that with SIP now. But with email, getting a bunch of spam can be handled in bulk, and letting a few slip by isn't disastrous. With phone calls, it could be much worse. As I understand, Google killed xmpp federation for precisely this reason.

Google killed XMPP federation because supporting open standards that allow people to use non-google services easily are bad for business.

You can say that, but Google has stated it's because federation was not used that much and that the spam aspect of it was the dominating usage. Which makes total sense and XMPP does not solve it, and in fact it's hard to see a solution for this problem that doesn't rely on a centralized solution.

Feel free to propose one. Because if you can solve it for XMPP, you can probably solve it for SMTP and become a billionaire for fixing spam.

Google can say that, but I and many others simply don't believe them.

Google is a business, and an advertising business at that. It's literally their job to try and make us believe what they say, regardless of the truth.