As reported by engadget,
In an attempt to curb cheating on the PC version of Destiny 2, Bungie is making SMS verification mandatory for free-to-play players. Starting today, if you want to take part in the game’s Trials of Osiris, Competitive Crucible and Iron Banner player versus player modes as a F2P user, you’ll need to verify your account before you can jump into any of them for the first time. When you try to join one of those modes, the game will prompt you to visit Bungie.net on your computer, sign into your account and enter the verification code Bungie sends to your phone. The requirement is designed to stop people from simply creating a new account after they’re banned for cheating.
SMS Validation is now live. All free-to-play PC players will now be required to verify their SMS prior to their participation in certain pinnacle level PvP activities.
More info: https://t.co/4Jdi9oNaII
— Bungie Help (@BungieHelp) December 9, 2020
SMS verification is a one-time process; you won’t have to go through the above steps every time you want to play Crucible, Trials or Iron Banner. If you purchased one of the game’s expansions, such as the recently released Beyond Light, you’ll have access to those Pinnacle activities with or without SMS verification enabled on your account.