Im not exactly sure the mechanics of it, but when a new number is activated (or a number changes carriers, etc) it pings some database that spammers have access to, and know that that number is live. Ive had it happen every time ive hanged carriers or set up a phone for someone. Dies down to normal levels in a couple months.
If youre ever onstage and need a hammer but dont have one, just use an SM58