Blizzard Support announces a phone number is required for Call of Duty: Modern Warfare II accounts as well as new Modern Warfare (2019) accounts. This "SMS Protect" feature caused enough problems for the recent launch of Overwatch 2 it was quickly scaled back. Word is such verification is also making its way into some Blizzard games:

Call of Duty: Modern Warfare II, 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.

In addition to the games mentioned above, some games require a phone number in order to use the in-game features like World of Warcraft Group Finder or unlocking the World of Warcraft 4 extra bag slots (requires both a phone number and Authenticator).

Limiting the number of free accounts that a single person can create helps keep players accountable for their actions and, in turn, reduces toxicity and cheating and ensures a positive community experience for all players.

If you already registered your phone number to play for free, this error means that you are logging in with the wrong email. Log out, make sure you enter the correct Battle.net email, and log in again.