Billing (No Respond since the last email send)

I have send a cancellation email to MTN and filled all the requested documents, till today I had never received a feedback saying that the contract is cancelled. Paid the outstanding balance as requested and only received a mail saying deactivation has been submitted for the 11th June 2020 and when I follow up I get no respond instead I get to be told that there's another outstanding amount for June that I need to pay when the contract ended in May. What is this for? I dont need to be blacklisted in the future as I make sure that all my accounts are up to date. I am not a person who like to complain on social media platforms but if this helps then I will use it