An alternative form of user authentication, such as 2FA/OTPs (One-time Passwords)

It’s an alternative form of user authentication, such as 2FA/OTPs (One-time Passwords), already used by OTTs (Whatsapp, TikTok, Uber etc) but instead of using the SMS to deliver the authentication/verification code, a missed call is used to deliver the code in the telephone number that issued the call. Instead of receiving a SMS, the user receives a missed call and the last 4-6 digits of the number that miss-called the user is the verification code.

Flash calls are the easiest way to verify someone's number that doesn't involve an SMS code.