This application sets forth techniques for managing phone number-based user accounts. According to some embodiments, a server computing device can be configured to carry out the steps of (1) receiving, from a first client computing device, a request to create a user account based on a phone number, (2) in response to authenticating that the first client computing device is associated with the phone number: creating the user account based on the phone number, creating a hidden contact address based on the phone number, and assigning the hidden contact address to the user account, and (3) in response to receiving a notification that a second client computing device is associated with the phone number: requiring the user account to be updated to be based on a different phone number, or assigning, to the user account, a visible contact address based on the phone number.