By default, when making internal calls, the telephone system does not provide any indication if the internal user you call is already on a call or set to do not disturb. Instead the call either just rings or goes to voicemail.
However, 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.
If you need status indication for a user without needing to make a call, you can add a 'user BLF' button to your phone.