Having a phone number isn’t proof of anything. Smurfs could just use family members’ numbers who would never play the game. It’s an extremely poor solution, as well as limiting to new players and an undesirable privacy issue. ActiBlizz does not need my phone #, and I don’t need them to have it and get hacked.
Do you have a source on that? Because I remember them saying it’s a problem when when people at higher ranks are intentionally deranking and smashing noobs (though the system is pretty good at bumping smurfs up where they should be/have them play against each other). Also, deranking is 100% reportable, and streamers have been actioned for it.