Complaint for an incorrect billing

Recently took out a contract with cell c the first month went well on my 2nd month they billed me incorrectly when I called to find out why . They say my contract was a fixed line initial but it has changed to an open line who authorized this because I didn't . Now they said they will investigate and get back to me after 48 hours until now nothing. I regret signing up with them . When there's other service providers out there . This is going to be a 24 months that's going to be a nightmare in my life . They don't care about their customers.