I live in France. I have heard several people say that if you are in the process of completing a transaction, for example taking out an insurance policy, signing up to an electricity supplier, mobile phone company etc, when you give them your RIB (ie bank account details) that means you have agreed to the contract, and it also gives them permission to begin taking payments from your bank account.
I know that to set up prélèvement (regular payment) they MUST get you to sign a mandate form, either on paper or electronically, though often this seems to be ignored. However, is it really true that simply giving your RIB means you agree to a contract, even if you have not signed one? I can find no legal references to this.