If you want to call cheap international with VM, you'll have to get one of the cheap Voips instead.
http://www.voipcheap.co.uk/en/index.html
90 freedays per topup, 300min per week of free calls
http://www.voipbuster.com/en/faq.html#23
With Voipbuster, you get 120 freedays per topup, but UK landline is not free
With Voip + freecall providers, you do need to shop around for the choice of free, and you will want to spend the credit eventually, as you have to keep topping up even if you only ever use the free destinations for the available time.