Using a phone number was just to present the idea, though there are a few things here I'm curious on.
I'm certainly not going to give out my cell phone to get potentially sold off and spammed with telemarketing phone calls and the like
There are agreements that can be created and are legally binding where the company is not allowed to share any phone information or include unsolicited communication. Riot has proven over the last five years that they do not abuse the email address associated with a given account, I can only assume this would be exactly the same for any other attached communication method.
so typically you'd get a home phone.
There is a generation of people that doesn't even know what a home phone is. The more important consideration here is the fact it limits the community to only people who have access to a phone.
Now assume that their accounts were linked through this system and TheOddOne decided he was done and was going to go out having fun, so he plays a bunch of troll picks, and throws games just to screw with people.
In such a system, OddOne would have a different account than MapleStreet. If one is banned, the other won't be affected.
As I mentioned above, I don't think using a phone number for authentication is necessarily a viable solution since it could potentially displace members of the League of Legends community that are already part of the game. I would, however, like to see some accountability when creating new accounts.