MTN **** Service Provider

MTN IS a **** company, I have a contract with them my data contract has expired I'm on month to month contract from the 26 may I've been calling 135 to cancel the contract they kept convincing me to move to a 99 package that gives 18GB day and 10GB night, I told them I'm no longer interested with MTN I'm moving to Telkom they kept dropping the call on me everytime I say I wanna move to Telkom. I even wrote to their complaints department no reply nothing till today. I'm wandering is it by force for me to have a contract with them even when I no longer want and my contract with them is finished why are they forcing me to stay???