The joke about the lack of mobile phones is perhaps starting to get a little tired, but now the fact is that it is starting to be needed even for non-mobile games. Blizzard stated that Overwatch 2 requires players to have a mobile number to play and now it appears that this also applies to the Call of Duty: Modern Warfare 2 reboot . It must of course also be a "real" number, prepaid cards and the like will not be accepted.
Requiring a mobile number for ranked play has been done before as both Dota 2 and Rainbow Six Siege have been running it since 2017. But requiring a number to access the game is relatively new. In any case, the idea is that it should help stop some of the toxic players out there.