Google is rolling out an update for the Android phone app that will warn you about spam callers. The update, currently for Nexus or Android One phones on Marshmallow, lets you block and report these numbers.
From Nexus on Google+:
Today, we're beginning to update your Google Phone app with spam protection on #Nexus and #AndroidOne devices to warn you about potential spam callers and give you the ability to block and report these numbers. If you already have Caller ID turned on, spam protection will be available on your phone once your app updates to the latest version.
Spam call protection will be turned on in the phone app by default. In addition to new incoming calls, you will also be able to mark old calls as spam in your Recent calls section in the app.