Go to the CX lounge. You have have access to the CX F lounge, but at the very back of the CX J lounge there is a business centre (walk passed the noodle bar and seating area as far as you can to the furthest wall), with a large and private phone booth which you can lock from the inside for extra privacy. There is a phone provided, plus you have plugs for power (laptop) and decent enough wifi depending on your method of calls. I found this a more than adequate place when I had a decent layover and the need to hold a few work calls.