Dota 2 will require a phone number to play in ranked matches

The new requirement is Valve's attempt to prevent players from using multiple accounts.

Do you enjoy playing ranked matches in Dota 2? Well, you better have a phone number ready because you're not going to be able to do that for much longer without one. Valve has announced in a blog post that it's now requiring all players to register a unique phone number if they want to play in ranked matches.

Valve is adding this requirement to reduce the number of players who use multiple accounts in ranked, saying it creates "a negative matchmaking experience at all skill brackets." Requiring a unique phone number for each ranked account will ideally prevent a noticeable number of players from doing this, resulting in a positive effect from players using their primary accounts.

If you're upset by this news and refuse to adhere to Valve's new rule, then you have until May 4 to enjoy all the ranked matchmaking you possibly can. After that, accounts without a unique phone number will not be eligible to participate in ranked matches. 

This is similar to what Valve has done with CS:GO's Prime Matchmaking, which offers higher quality matchmaking if you've registered a phone number. However, in that game, regular ranked play is still available without a phone.

Removing a number from your Dota 2 account after registration brings about a couple things. You can add a new number right away, but the number that was removed will not be usable for three months. Valve says this is to prevent someone from removing a number and immediately registering it with their alternate account. On top of that, Valve is not allowing numbers from online services to be registered. That means you likely won't be able to use your Google Voice number.

You can read the full blog post here, which goes into a number of other changes, including party changes, the return of ranked solo queue, and improved handling of nefarious behavior.