In an effort to combat smurfing (more on that in a sec), Valve has implemented a new feature for Dota 2 -- if you want to play ranked mode (the be all end all for pros and upcoming pros), you'll need to give them your phone number to authenticate your account. This change will take place on May 4, so get yourself situated.
For years games that are highly predicated on ranked tiers have been trying to figure out ways to combat the act, which very simply involves creating a brand new account in the effort to play outside of the matchmaking ranked imposed on your original account. In other words, pro players can register a smurf, then join a "newbie" queue and stomp people of lower skill level. It's not just for pros either, as some players who contend they're stuck in "ELO hell," (read: low ranks where they feel they don't belong, playing with others who are below their skill level) often smurf so they can attempt to place in a higher rank, then abandon their original account.
As someone who has known many smurfers in their time (the practice never really interested me, as I typically place high enough in ranked where I'm happy), I'm torn on the issue. On one hand, it really sucks to be a new player in a lower league, trying to make your way up, only to find that you're actually battling a smurfed Grandmaster top 1% player on the other side -- it deters the hell out of you from climbing the ladder. On the other, people who do smurf contend that they can play the game any way they want, and they should be able to make a new account as it doesn't break any major rules (and they frequently spend money on burner accounts to progress quicker).
Ultimately it's up to the content provider to decide how they want to enforce their terms of service (TOS). For some games like Heroes of the Storm, Blizzard has a lax view of the practice and doesn't really enforce any sanctions. League of Legends has sort of a middle of the road view on it, allowing it but making some exceptions, and now, Dota 2 is directly fighting it.
It isn't going to step folks from getting a burner phone for $5 (or creating a Google Voice/VOIP number) and smurfing, but it's a marked attempt to try. Like most Valve ultimatums, it'll probably end up failing and they'll retract it in time.
Matchmaking Update [Dota]