Which is a move by Valve to combat players will multiple accounts taking part in what's commonly referred to as "smurfing". A MOBA staple seen in a variety of free-to-play titles where secondary and third accounts are treated in a less serious fashion than someone's primary, or utilised in a way to boost lower level friends to higher ranked tiers. Generally, unfair stuff all round.
Asking for a phone number in order to make players eligible to take part in Dota 2 ranked play may sound extreme, but it could be the only way to combat this phenomenon. A two week grace period for players to link phone numbers with their Steam Accounts is currently in effect.
This announcement comes as part of a larger update to matchmaking, which you can read all about here
. One of the cool new features is the introduction of proper Solo Queuing whereby players will only be matched up against and with other Solo Players. Very cool.