Poor service I got to cancel my contract

Poor service I got just to cancel my cell c Sim card data, I had a phone contract with them, the phone contract end however they didn't notify me about the Sim card data that was left uncancelled. I told them to cancel it and they told me my statement is on arrears, without even sending me the statements, I paid the arrears but yet they still, call me to harass to pay money that I already paid just because they don't want me to cancel. I am very disappointed, I even sent them proof that I have paying but still they keep calling for money