How do smartphones decide whether a phone call is spam or not?

Like everyone else, I'm receiving spam calls. With my current job, most communication goes over the Internet (online meetings, chats, etc.) so in my case, it's nearly only spam. What I'm wondering is how can the phone decide whether a call is a spam call? It can't be based on the phone number since 99% of the time (it seems) those are spoofed. Once in a while, I get calls from people who received a spam call with my phone number spoofed. I've heard a year or so ago that the TELCO were going to implement a system to help verify phone numbers, but I've not seen anything more than that.