Source: Bungie
As Bungie preps for Destiny 2's crossplay support, the developer is rolling out SMS verification on the PC. It's a mandatory step for free-to-play gamers who want to play the game's Trials of Osiris, Competitive Crucible, and Iron Banner in player versus player modes. If you haven't done it yet, you'll need to verify your account before jumping into any of the modes.
The game will prompt you to visit Bungie.net on your computer when joining one of these modes. You'll need to sign in to your account and enter the verification code you'll receive from Bungie on your phone. It'll help prevent players from creating new accounts after they've been banned for cheating.
SMS verification will be a one-time process, so you don't have to do this every time you play. If you purchased one of the game's expansions, you'd have access to these Pinnacle activities with or without SMS verification enabled on your account.
Source: Engadget