The most popular way to implement 2FA is to receive a text message once a password has been provided for account access. During the registration process, a phone number is asked for. The function of the phone number is to act as the 2FA in the future. Depending on the service, the SMS code may be sent via text or automated voice call.
Although there are other types of two-factor authentication used by organizations and individuals, the text-message-based one is the most common. Essentially, users input a password to a given account which generates a prompt to input another code sent via SMS to the user’s registered mobile device. The code is for one-time use only.