This one may make you smile. Three have a new android VOIP app which if you cannot get a signal and have an android phone will allow you to make calls over WiFi if avaible, its called Three In Touch android app. At the moment its in beta and you have to call Three to get a link via text to download the android apps it is not in the Google Play store. Now the great bit, if you use the android app and make calls over WiFi it still comes out of your minutes allocation, so Three presumably because its routing over then internet until the last mile the costs to Three are minimal but cost to you is the same..........
Android users on Three might still want to check out the Android app when it comes out of beta.