Once Local Number Portability (LNP) is available for an NXX, carriers must "query" all interswitch calls to that NXX, as appropriate, to determine whether the terminating customer has ported the telephone number.

• Carriers will accomplish this by sending a signal over the Signaling System 7 (SS7) network to retrieve, from an SCP or STP, the LRN associated with the called telephone number, if the number has been ported.