To combat cheating.
Cheating is rampant on Destiny 2 on PC in no small part due to the game being free and thus completely accessible to creating dozens of burner accounts. In order to combat some of the cheating Bungie has instituted an SMS verification on certain PvP activities that went live earlier today.
The activities are Trials of Osiris, Competitive Crucible, and Iron Banner and it only applies to free to play accounts. If your account owns any of the expansions will be able to bypass this requirement.