watsapp call does not work with O2 network

hello 

I have an o2 DSL since june but my wats app call does not work. Google hangout works.

Can you tell me any settings I need to do on routers.

I have o2 Home box 6641

only dsl connection, NO LANDLINE is there.

There is nothing in Port Forwarding. Can you tell me steps to enable watsapp calls