It seems as though Call of Duty: Modern Warfare 2 players will be required to connect a phone number to their Battle.net account before playing the game.
That's according to notice posted to the Blizzard support page which notes two-factor authentication, called SMS Protect, is a requirement to play the shooter (thanks, PC Gamer).
"Call of Duty: Modern Warfare 2, newly created Overwatch 2 accounts, and newly created Call of Duty: Modern Warfare accounts, require that you add a phone number to your Battle.net account."