Email spam became tractable on the end user side with domain and IP address risk scoring. Caller IDs are so easily spoofable it's like open relay email servers of the past.

When they spoof are they using otherwise valid numbers? That is, if you returned the call you'd speak to someone's grandmother?

None the less, can't the phone providers detect the excessive outgoing traffic? And if it's a residential number can't that raise a red flag?

Do you have a link for that? I have an Android phone, but I received a call marked as spam yesterday. I'd love to be able to just block those.