MTN triple charging a canceled contract

After sending emails to MTN from the 1st of March with their "notice to cancel a contract". April the 17th I get an email billed almost 3x the amount for router data. When I call their 135 number, it's impossible to get through and when I send an email to customer service and retentions- customer service forwards the email to retentions and when retentions call me, I am told only customer service and billing can help me. I am transfered- I get "my system is slow" or the phone is never clear. I put my notice in time and I am getting these messages that do not make sense. I did not agree to any 3x amount and you are bullying us into bad credit record.