A switch, owned by the phone company, establishes connections between callers and the people or computers they are calling. Switches are connected via high-capacity lines, almost all digital, to other switches. Each ISDN connection goes from a user to a local switch. If users call someone who is connected to a different switch, their local switch establishes a connection to the destination switch, sometimes through one or more intermediate switches. The destination switch then makes the final connection to the called party's line.
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