There is a new type of account authorization in the PC version of the CoD: Warzone - a new account must be confirmed with a password received in an SMS message before use. The change is aimed at reducing the number of cheaters and if you believe the opinions on the forums, its introduction has met with wide disapproval of cheaters.

IN A NUTSHELL:
A new authorization system has been introduced in Call of Duty: Warzone, which requires confirmation of the newly created account with a code received in an SMS message. The change applies to users playing on PC and is aimed at reducing the number of cheaters spoiling the fun of honest players in Infinity Ward and Raven Softwar's game.
It seems that the developers have achieved the desired effect - there were a lot of complaints on the forums dedicated to cheats. Here is one of such statements (via Reddit).
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