In your User profile, you can see if a colleague's extension number is busy on a call.
In the navigation bar on the left-hand side of your screen, you can find the Presence section.
Here you'll find all internal speed dial/Extension numbers. Clicking them can be used to start a call to a colleague's sipgate extension.
If an extension is already on a call, the square for that number will appear red.
If the extension is currently being dialled, then the square will have red borders and the User's name will move.
You can still start a call to a busy extension.