What do the different colored circles next to my colleagues' statuses mean?

A green icon indicates that the person is available for phone calls.

A yellow icon indicates that the person's telephone is currently ringing or that the person is making an outgoing call but that this has not yet been answered.

A red icon means that the person is currently on a call.

 A grey icon is displayed if the person has activated a notification profile and is not currently available to take calls. If you hold the mouse over the person's name, the display shows for how long the profile will be active and any information messages that have been issued by the person.