In order for an agreement to terminate an existing contract to be binding, what requirement must it comply with?

There are a number of ways by which parties may terminate a contract by agreement. Which of the following is NOT one of these ways?

Paul and Grace entered into a contract for the provision of cleaning services. A term of the contract stated that if either party decided to terminate the contract, notice must be given to the other party in the form of a written document no later than 14 days prior to the intended termination. Grace decides to quit cleaning and terminate the contract. She telephones Paul and informs him of her decision 15 days prior to the date she intends to stop. Is the termination valid?