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.