Complain telecom

Good day Am here to complain about my contract ever since they over charge bon 2019 and I wrote to you guys non of you come back.I sign contract for my modern for R139.00 per month and after 6 month the send me the outstanding balance I paid from there I keep calling customer care without any help .also I paid deposit of R900 .I didn't use that modern since they overcharged me for almost 2 years now,this month they send me invoice to pay over R2400 for what and still have my deposit. I want this issue to be resolved as soon as possible and to tell those customer to stop harassed me with their call to go pay for the services I never use.