Services like Signal and WhatsApp can user 3P services that allow them to be notified when a phone number is rotated (given to a new user). They should ideally be doing this, I cannot verify if they are or not.
Second, Signal and other services have implemented secondary registration requirements such as a PIN, which they will require during a new device install or at other times.
Third, you can build models or crude business logic to identify when a number no longer appears used for a period of time. Carriers do not reassign a number immediately. Assigning a number thus one user cancelled, to another user, is seldom done before a 90 day hibernation period.
Second, Signal and other services have implemented secondary registration requirements such as a PIN, which they will require during a new device install or at other times.
Third, you can build models or crude business logic to identify when a number no longer appears used for a period of time. Carriers do not reassign a number immediately. Assigning a number thus one user cancelled, to another user, is seldom done before a 90 day hibernation period.
I used to work at a cell provider.