Giffgaff is a new low-cost UK mobile network. It's a virtual operator, which means it uses o2's infrastructure and its SIM cards will work in phones locked to the o2 network. Cross-network UK calls cost just 8p per minute and are free to other giffgaff numbers, and international texts are only 8p each to several countries. What's more, you can earn cash by helping other users on the official site http://giffgaff.com and for promoting the network!