O2 problems

I am really dissatisfied with o2 actually. I am not happy with their service and I would like to cancel all contracts with them. I am saying from the point of no return. I have been a loyal customer to o2 and look what have they done to me?

I called this morning to inform them that they have over charged me and they insisted that they have not done so because o2 have recently change their plan and I would have to stick to it. Well then, what can I say about it? I don't care about o2 anymore. I think it is better to stick with Vodafone. I know that Vodafone is more expensive than o2 but they do provide me with good service. All my text messages have been received by other people and I have received all my friends' text messages. What is the matter with o2? I hope someone will give me an answer for that. Where is the level of service which o2 had before? Are they in some sort of trouble now?