(I apologize if my english isn't 100%, i'm not a native speaker)

When I've queue'd up competitive I often once per a couple of games I get a guy on my team(or multiple) being really toxic in voice or text chat. When you just tell them to calm down they will yell slurs and crap in voice while also trashing your rank.
And if you comment on it by adding that you will report them and mute they go "Bro I don't care, this is my alt account"(this is from personal experience in Plat elo).

I feel like forcing people to add their phone number will actually force you to behave, you can still get multiple accounts with the same phone number. Only now your ban will actually transfer if its the same phone number attached.

This is not perfect in anyway, but its a step in the right direction.