Still receiving sms messages on paid up account

I have paid my x-rays account in full in May 2015, sending proof of payments on various occasions. I still receive messages that I owe money on this account. Last week I phoned, the lady I spoke to said they will phone me back, no one returned my call. On 6 October 2015 I received a message that I owe money. Today, 8 October 2015, I received another sms claiming that I owe R 304.10. I once again phoned, this time I was told I owe R 154.10, as they are using two different systems. How on earth can you run a business using two different systems that shows different amounts owing. The account was settled in full on 25 May 2015, there is absolutely no amount outstanding. I was told that they will give the information to the credit department, and if they are able to locate the payment, they will allocate it to my account. I am furious!!