This system was introduced to limit the number of free accounts a player can create in hopes of curbing cheating. Usually, when a player is banned for cheating or bad behaviour, they can easily create a new account and go back to playing the game within minutes. With this system, they will have to potentially face the consequences of their actions, which will hopefully cut down on such tactics.
The company is standing behind the decision to restrict the phone number requirement to postpaid users only, as it prevents users from getting a prepaid burner phone or a VoIP online number and circumvent this system. They are also ensuring that the system is applied to all their new games, including Call of Duty Modern Warfare II and Overwatch 2, among others.
These are not the first games to feature such a security measure, as Dota 2 and Rainbow Six Siege also require similar mobile verifications, with the difference being that they only do so for ranked play. Thankfully, they don't have the same bug restricting users to only using postpaid phone numbers.
Only time will tell if or when the company will fix this supposed bug or whether it will also be implemented on Steam accounts as well. For now, they are saying that they will relax the requirements for Overwatch 2 players who already used their Battle.net account to play the original Overwatch. Let us wait and see if similar concessions are made for Call of Duty players as well.
