If you have an Android device, you can disable automatic phone number verification, which should prevent Google from linking sign-in accounts together. You can find this toggle in your Settings app under Google or Google services and preferences > All Services > Privacy & Security > Phone number verification. (Note that the exact path may differ by device.) You should periodically check to ensure this toggle remains off.

Updating this setting does not disconnect phone numbers and Google accounts that have already been linked. You can find out which are which via your Google Account Personal Info page. Select your profile picture in the upper-right corner to go through each signed-in account and open the Phone section to select and delete any numbers you don't want associated.

Finally, Malwarebytes advises scrolling down to the Usage section on the Phone page to view which services are listed as well as reviewing your Security & sign-in settings. If you added your phone number for two-step verification or to a separate app like Google Pay, deleting it from the Phone page won't remove it entirely from your account, and you'll need to update those settings separately. Automatic phone verification shouldn't be the culprit here, but it's useful to audit what your number is being used for and on which accounts if you want to keep things separate.