I have just inherited my daughter's iPhone 3GS when she upgraded to an iPhone 4!! I am like a kid with a new toy, but do not want to be hit by high call charges so use PAYG from O2 in UK.
I live in France so am conscious of potentially high call charges so have downloaded one of the Apps called VoipDiscount. I invested 10€ up front and then (provided I am in a WiFi zone) I can make free calls to landlines for three months!! At the end of the free usage period my call costs are deducted, as are calls to mobiles (at a low rate) from my 10€. As soon as you top up your three months of free calls starts again.
I feel certain the this app is available for Android and Blackberry systems.
Certainly worth a try. I have been most impressed with VoipDiscount.