Extension User on a call help....

Hello and advance thanks.

Is anyone able to advise please.

We have the phone system as below.  What we cant figure out is how to set this up so when one of the extensions is on a call - its shows on the system that extension is in use.

Whilst when, for example, I am on a call, I can see that a staff member is trying to call me, however I cant end the call with a client to take the internal call, so need the system to show other users my extension is in use. 

Help!!