By default, when making internal calls, the telephone system does not provide any indication if the internal user you call is available, already on a call or set to do not disturb. Instead the call either just rings or goes to voicemail.
However, there are various methods by which you can get some indication of their status:
If you need status indication for a user without needing to make a call, you can add a 'user ' button to your phone. See Programmable Feature Buttons
Other users can configure an absence message. If they have done that, the message is displayed on your phone when you call them. To set your own absence message, you can user a programmable set to the Absence Textbutton. See Programmable Feature Buttons.
Your system administrator can enable busy indication for selected users or for the whole system. When enabled, calling those users displays Do Not Disturb or On Another Call on your phone if the call is not diverted to voicemail.