There are loads of cheap services that bounce your mobile calls via VOIP so you only pay the cost of a local call on your phone bill, and 7-10p minute to the provider.
I use rebtel (do a search) which seems pretty reliable, although I sometimes have to call 2 or 3 times to get a connection. It means I can talk to my fiancee for 20 minutes for the price of a Starbucks Cappucino
