* Problem: Spammers automate creation of accounts. Solution: Reuse the MFA infrastructure as some kind of "CAPTCHA". The phone number is not stored.
* Problem: Spammers use a single phone number to unlock 1000's of accounts. Solution: Store the phone number - so those kinds of misuse can be detected.
* Problem: Ads-Team wants to sell more targeted ads. Solution: There is possibly a phone number stored in the user profile, use that.
Who is to blame here? The Ads team that didn't check if the number can be used?