If the client is upset, be patient. be a good listener. once they are done venting, be helpful and sympathetic. Guide them and help them with their query. this will calm them down and make them friendly. once they are calm, start with your pitch in a way that doesn't seem like you're trying to get them to buy your product. ask yes questions. The client will surely convert if its a sales call and if its a call to prevent cancellation, then the client will surely not cancel. Its a game of words