Starting on July 7, 2020, Google will make phone verification prompts the primary 2-Step Verification (2SV) method for all eligible users, unless they are already using security keys as their 2SV method of choice. This means that if you sign in to your Google account and are also signed in on a smartphone, you will be asked to follow phone prompts to verify the login attempt. This will help increase account security while making it easier to sign in.
This won’t apply if you use a security key to protect your account. You’ll also still be able to use other methods (such as a code received by text) by selecting a different method during the phone prompt verification steps.
For full details see: https://gsuiteupdates.googleblog.com/2020/06/stronger-google-account-verification-phone-prompts.html