Incorrect billing

I am fuming as I am writing this. I signed up for 20GB with Telkom for R249. All was fine until April where I receieved an Invoice for R749 I called Telkom was given senseless explanation about limit which i never signedup for and never used it. Lady I spoke said she is takin the limit off. We are in May I am still getting R749. I want to cancel the contract with them they are ****** and opportunists during this pandemic.