Hi @Anonymous and Welcome to the forum.
We are all customers here like you.
Without any background from your post at this point I would just like to post a post with generic solutions in it. If you've been through these type of possible resolutions then I apologise.
http://community.o2.co.uk/t5/Pay-Monthly-and-Pay-Go/Self-help-guide-Is-the-network-down-for-me-or-everyone/m-p/593752#M59833Other questions would be ;
Does this issue happen everywhere.
Have you tried another phone as a process of elimination.
Do other people on O2 around you have the same issue.
What have O2 said to you.
What does your Service Status say.
More info will help us to try and help you.
Take care.