The whole reason this was made necessary was to combat players who use multiple accounts to wreck other’s ranked matches. This is called “smurfing” or creating secondary accounts to play on is quite a common thing in free-to-play competitive games. However, in Dota 2 it can result in players tanking ranked matches without much consequence.
Starting today for the next two weeks, players have to register unique phone numbers to their Steam account. Starting 4 May, those without will not be eligible to compete on the ranked ladder. Ouch.
It’s not the only change with the update either. There’s also an option for solo queue players to match up with other solo players, alongside low-priority duration based bans which will apply to ranked matchmaking. Last but not least, underpopulated servers known for being used for artificially boosting matchmaking rating won’t be offering ranked matchmaking anymore in an effort to push players into more populated servers.
You may read the full list of changes here.