Summary
Google says Android verifies phone numbers by default and adds them to every Google Account signed in on the device. The setting can be managed in Android’s device settings.
Google’s Android Help documentation says phone number verification is on by default when an Android device is set up. When enabled, Google verifies the device’s phone numbers and adds them to all Google Accounts signed in on that device—including accounts already present and ones added later.
That means adding a Google Account to the device can link it to the phone numbers already on the device. Adding a new phone number can also add it to every account signed in there. The setting is managed on the device, rather than separately for each account.
Why Android verifies numbers
Google lists several features that phone number verification can support: signing in to an account with a phone number, recovering access after sign-in problems, emergency location services, and finding or remotely locking a lost device. The numbers can also support RCS messaging, video calls and Quick Share.
Google and the mobile carrier may periodically reverify numbers to check that they still belong to the user; carrier charges may apply. The help page does not give a specific re-verification schedule.
How to manage verification and sharing
To change the device-wide verification setting, open Settings on the Android device, go to Google > All services, then find Phone number verification under Privacy & security. Google says verification can be turned off there.
Google also describes a related setting called Better sharing on Google. It turns on when phone number verification is enabled and supports Google services that facilitate calls, messages and notifications for photos and other content sent to the number. If multiple Google Accounts are signed in, Better sharing is turned on for only one of them.
That sharing setting has its own control: in Settings, select the relevant Google Account, open Manage your Google Account > Personal info, then go to Contact info > Phone, select the number and change Better sharing. Google says additional settings are available to control how each phone number is used.