According to a Battle.net article, Call of Duty: Modern Warfare 2 will use the same SMS protection feature found in Overwatch 2.
While the idea is to prevent people from creating low-level accounts to farm less-skilled players and cheaters, it will also block VOIP, certain countries, and pre-paid phone lines, potentially excluding a good number of players. Despite this, the SMS protect program had more than a few issues, resulting in Blizzard Entertainment taking a step back.
As of now, players are up in arms about the decision. Some have already noted that other platforms, such as Steam, have looser requirements and suggest players use that instead. A fair amount has also criticized the choice, as Overwatch 2 is free-to-play, whereas Call of Duty: Modern Warfare 2 is paid.
Activision has not released an official statement, but given the negative buzz, we might see them quietly drop it or step back like Overwatch 2 did.