The way I was initially picturing it would involve Signal keeping track of the salt they assigned to the contact number, with that specific salt maybe 'expiring' after a certain number of days (to get rid of salts where someone registered with Signal, didn't like it, and never registered again). After thinking about it some more, I realized that this would probably lead OWS to collecting big lists of registered users and their contact patterns, which goes against their policy of trying to retain as little data as possible.