To call another person in your business, lift the handset and dial the other person's extension. This is typically a 3, 4 or 5 digit number, and may begin with #.
Note that depending on your phone type and configuration, the call may not complete automatically once you have finished dialling. For example, you may need to hit the OK or Confirm key once you have dialled the number.
If you dial someone within your business, your telephone system may be configured to show you the name of the person you are calling once their line starts ringing so that you can check you have dialled the right number.