contract ended but still billed

My contract ended in May already, and on the 8th May I called your call centre and spoke to Julia to migrate the number back to prepaid and was advised it would be done in 5 days, however this has not happened and I have sent you an email as well. Calling the call centre has not been fruitful either one holds for over 20mins so I have given up calling the call centre. And this month I have been billed almost double the amount I used to pay and this don't make sense. Please cancel my contract and also remove me from your database am tired of receiving calls from your sales department regarding renewing as I have told them many times that I am not interested. Please I need this contract cancelled as in yersterday.