Techniques are described for better ensuring that calls or transmissions directed to a subscriber or endpoint to which a number was previously assigned are not received by a new subscriber or endpoint. According to one class of implementations, a telephony platform maintains a registry that associates a secondary identifier with each phone number that is currently assigned to a subscriber. The identifier may point to and/or include any of a variety of information that maps to the current subscriber. When a calling party initiates a call to an assigned number, an additional identifier is transmitted with the call routing information that corresponds to the entity or endpoint to which the call is directed. The telephony platform makes call routing decisions based on the correspondence of this information with the information maintained in the registry.