Google Prompt is an authentication option launched in June 2016 as a more secure alternative to receiving codes via SMS. Prompt is primarily aimed at overcoming the growing insecurity of SMS codes. With prompt, data is still being sent back and forth, but using an encrypted channel. As long as the phone is within reach of a data connection, the user will also receive a real time warning every time someone anyone attempts to log into their Google account. The other advantage of this is that its quicker to hit yes when prompted than it is to enter an SMS code or, in the case of Google Authenticator, a six-digit code.”]
Source: https://nakedsecurity.sophos.com/2017/07/18/google-wants-you-to-bid-farewell-to-sms-authentication/