PAYMENT NOT ALLOCATED

I have paid my account on the 23rd April an amount of R260 to settle my account, I used the Ussd number as I always do. I was surprised to receive a statement stating that I am in one month arrears, I then send an email immediately to notify that I have paid and even attached my bank statement. I send an email on the 27th May and I did not receive feedback from them yet I was told to make payment by the 31st May. I am not happy with their service and this will affect my credit score and it will mess up lot of things for me. All I want is for the payment to be allocated then close the account.