7 Please try a manual roam.
1) Go to your phones "settings".
2)Select mobile network options.
3) Change the Network Selection Mode from Automatic to Manual
A list of available networks will be displayed after a few moments.
4) Select any other network that is not your home provider network. After a few moments you will see "No Service" on the display.
5)Now select your home provider(which is o2) network and change the selection back to Automatic.