The Telephone
  Information Tones
  A Basic Call
  Subscriber Line Network
  The Local Exchange
  Connection & Disconnection of a Call
   Charging Functions
   Analogue-Digital Conversion
   Operation & Maintenance
   The Control Function
   Power Supply
   Remote Functions
  Other Types of Exchange
   The Transit Exchange
   Both Local & Transit Exchange
   International Exchange
   Exchange Networking
 
 
 
 
 
 

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