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The purpose of *18 (or *350) is to help cellular base stations find your cellular phone while roaming, so you can access your voice mail by calling your own number. This is "FMR", for "Follow Me Roaming". In older cellular systems, cellular switches weren't very smart. If you called your own wireless number while roaming, the system would find you and try to deliver the call. But you're on the phone calling your own number, therefore it would return a "busy" signal. The called system didn't know to tell the calling system to return your call to voice mail. So entering *19 before calling your own number, turned off Follow Me Roaming, preventing the cellular network from looking for you beyond your home market. *18 turned it back on. Today's cellular switching systems are smarter, but some carriers still recognize *18 as a valid command. Some cellular carriers still allow *19 to stop incoming calls while roaming, and need to keep *18 active to undo it. Some of them recognize it even if it doesn't do anything. Sometimes if a customer is having problems receiving incoming calls, their carrier's Customer Service recommends entering *18, in case they accidentally pressed *19. *18 is valid in "B" Channel Cellular-band (850 MHz) carriers. *350 is for "A" Channel Cellular-band carriers. It was never used with PCS (1900 MHz) or GSM carriers.
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