Deducts interes after account has been settled

I settled my student loan account on 15 February. I received the sms and email that the account balance is 0.00. Still Standard Bank deducted interest in March and April. I phoned the call centre where the lady said it is a mistake and that she can see that the account was settled. She said she will send a settlement letter, close the account and refund interest deducted. Still I have not received a settlement letter or any confirmation that this is processed. When I phoned the call centre again the call drops or I keep online without anyone from collections answering. Please help.