Cheapest way to call Australia from the UK?

My sister and family just emigrated to oz - what's the cheapest and easiest way of keeping in touch with them on the phone. I've looked at the BT International add ons (�1 a month for 3p a min calls) - are there any other better deals?? I don't really understand internet phones yet or what you have to do or have to set one up. How do they work how effective are they? Would I use one in conjunction with a webcam?? What sort of price are webcams and internet phones? Where is the best place to get them? How simple are they to set up? We're with BT (phone) AOL (INTERNET) Any advice? Thanks