You won’t be able to join any ranked match if you don’t do that.
The news comes from the Dota 2’s blog and all people that don’t like to play fair won’t be too happy. It’s a part of the Matchmaking Update and starting from April 20th players who want to join Ranked matches must have a unique phone number added to their account.
Players using multiple accounts create a negative matchmaking experience at all skill brackets, so our goal is to add just enough friction to this process that the number of players doing this will be noticeably reduced. Having more players using their primary accounts will have a positive effect on both Ranked and Unranked Matchmaking.
For the next two weeks Ranked matches will allow to play both with and without the phone number added, but the grace period will end on May 4th. You will be able to change the phone number, but when one is removed it can’t be added to any account for the next three months.
There are also other changes to the matchmaking coming, including Solo Queue, better detection of botting users and some regional changes. Details on Valve’s blog.