You are having problems with sending or receiving SMS/texts then this trick might resolve the problem.
1. Go to settings 2. Then go to mobile networks 3. Select a network other than O2 (gg) i.e orange or tmobile. 4. Come out of settings and it should say no network coverage or something similar. 5. Repeat the process this time choosing O2 (gg) which should update the message centre and hopefully restart the texts.
The message centre number in your message settings should be +447802002606
On an iphone then it's a slightly different process.
1. Go to settings 2. Go to carriers 3. Switch off automatic and manually change the carrier to something other then O2-UK i.e vodafone or orange. 4. Come out of settings and wait until it fails to connect to the other carrier. 5. Once it fails repeat the process and this time choose O2-UK
For a shortened link to this guide click below http://is.gd/manualroam
And maybe worth trying a reboot/restart your phone!