Account closure service failure

I had logged an account closure on the 29th March 2021. However, I had a credit on my account. When logging the closure I was informed that I should send an email to *********** to have my credit refunded, and my account will be closed. I had followed the process I was advised on, however, my account was not closed. I have now been receiving collections calls for an unpaid account that I haven't used in over a year. when called the call centre the agent told me that they will log the closure and have the account closed as all that was incurred was service fees, this has still not happened.

PLEASE SORT OUT YOUR SERVICE FAILURE, AND CLOSE THIS ACCOUNT.

My credit record has also been impacted by this. SORT OUT THIS MESS ASAP!!!