What is the difference between PBX and PSTN?

• PBX (Private Branch Exchange) is used in office environments where 9 must be dialed to call an outside line (outside of the office building).
  
  PSTN (Public Switched Telephone Network) is a standard phone line, similar to the one in your home.
  
  The default Line Type setting is PSTN. Follow these steps to change it to PBX:
  1. Press the Setup button.
  2. Press the or button to select Fax Settings, then press the button.
  3. Press the or button to select Basic Settings, then press the button.
  4. Press the or button to select Line Type, then press the button.
  5. Press the or button to select PBX, then press the button.
  6. Press Setup to exit the menu.