please close the account!!!

I phoned FNB on 03/09/15 and asked the settlement amount on my credit and was told and I paid within 10 mins of the call and was told to write an email requesting for closure which I sent within 30 min of the call and two weeks later was called by FNB to confirm closure of the account and was promised a letter will be sent to confim it has been closed. and I never got the letter, yesterday I got an sms saying to pay my credit card because its due.. how is this possible when I have paid the settlement amount I was told. and sent the letter.