Maybe this is not quite in the spirit of these forums but go here: click here
This site gives the cheapest call rates from the UK. For example, UK to Australia is 0.5 pence per minute.
When in Australia, call the UK, give them your number and ask them to call you back. The number will show on their phone bill and you an slip them a quid or two when you return.
Australia 0.5 pence per minute
New Zealand 0.5 pence per minute
South Africa 2.0 pence per minute