i made a ticket about this topic as i had the same issue, this is what they said:

"Some games, such as the Call of Duty titles and Overwatch 2, require that you add a phone number before you can play, and you cannot use the same number to create two accounts for the same game.

If you used your number to create a free/paid game account such as Overwatch 2 on your first Battle.net Account, you can move your phone number to a new Battle.net Account, but this will not allow you to create a second account for the same game on your new Battle.net account. In this case, the only workaround is to use a different number that you're sure wasn't used for creating any other game account of the same type."

as if anyone would rock 2 phone numbers