Everytime you want to cancel a contract or give a notice (your lease, your insurance, your mobile phone...) you MUST send a « lettre recommandée avec accusé de réception ». It's the easiest legal proof. Don't forget to send it in on time. Look at the section of your contract that specifies « résiliation du contrat » (cancellation). Be careful with insurance contracts because you often need to send your notice three months before the time you want to cancel your contract. This waiting period is called a « préavis ». Be careful, in France it can be very long. And if you send your notice too late, the insurance company can refuse it and force you to pay for one more year.
When you send a « lettre recomandée » keep the paper the postman gives you. It's the
legal proof of your action. Then as the sender, a few days later you'll receive another paper in your mailbox (same size as the form you filled in out) : « recommandé, accusé de réception ». At the back of this paper there's a big « AR ». KEEP this paper. It's the legal proof that the letter was handed to the person it was addressed to. You must keep this paper until your contract is officially
One more thing. Read your contract carefully (even if it's all in French and difficult to understand).
Look at the section of your contract that specifies « résiliation du contrat » (cancellation). If you see « reconduction tacite » it means « renewal by tacit agreement ».
If you have to cancel your mobile phone contract.