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       Private Branch eXchange In principal this is 
      much the same as PSTN but for a private telephone network such as within a 
      company. Can be a simple hardware device. The advantage to a company 
      running a PBX is that outside lines don't need to be run to each desk, you 
      are probably familiar with 'dial 9 for an outside line' this is the PBX at 
      work. With phones connected to the PBX internal calls (within the company 
      - therefore within the PBX) are free, only external calls are chargeable.    |