IS41 is an SS7 application service that allows cellular users to roam into other provider's networks. It makes use of the HLR and VLR to keep track of the user's location and allows calls to be placed/recieved. Every cellular user has a HLR that contains the current location of the user. This is updated even when the user moves into another coverage area through use of the VLR. The roamed-on network sends a VLR to the HLR to inform it where the user is. The HLR will user this information to direct calls to/from the user.

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