The Make call button is activated when you do not have any contacts in progress. The button is used when you want to make a call yourself, either externally or internally to another agent, opening up Unifinder. See Make call and enquiry via Unifinder
In the opened Unifinder support function you can search for another agent using various search criteria. Alternatively you can enter a telephone number directly, pick from a list with the last entered numbers and saved favourite numbers, or fetch a number from the address book.