When does the requirement for written form apply?

If an operator contacts you on their own initiative via phone to enter into an agreement, you must accept the offer in writing for it to become a valid agreement. Note that this only applies if you sign the agreement as a consumer; for business agreements, there is no requirement for writing. If you contact the operator, or if you ask the operator to contact you, there is no requirement for writing. However, it is not allowed for the operator to circumvent the requirement by calling you and asking you to call back.