marquise: It was agreed that I would have illimited Internet access and I don't. We agreed that my evening and week-end calls would be free but they are not etc.
Once you are over 18 it is generally accepted that if you say "I agree to an 18 month contract" then you mean it. However, as I stated previously, if there is a problem you can ask them to put things right and then, if they don't, you can end the contract. You do not have much reason to take the provider to court but, if you stop paying, they may well take you to court and that is where you can put your case. Before you stop paying, I would advise that you take the following steps.
Write (don't phone), giving full details of the problems, related communications and any action taken. Your letter should request that the problems are rectified within 30 days. If they do not put things right, your next letter should state that you believe that they are in breach of the contract and, unless they start to fulfil thir side of the agreement within the next 30 days, you will cease payments.
The likely outcome of the above course of action is that the alleged debt will be passed to debt collectors. You should state in writing that you dispute the debt but would be happy for the matter to be passed to a local court and decided there. If it goes to court, you should attend, take copies of your letters, posting receipts and any other relevant documents and you should put your case there. It is possible that, if you state the reasons for disputing the debt and you state that you would like the matter to be decided by a court, then the provider may well not take things any further.