Account paid but get told I am in arrears

I pay my account every month. Actually more than I should on my installment and this month I get an sms nearly everyday, telling me that my account is in arrears. I received a phone call and was told that I must send the proof of payment as it was via EFT. I did so and I received an email back telling me that the payment was found and will be allocated correctly. I thought everything was fine until I received a phone call again last week telling me that my account is in arrears and that my credit status is going to be affected as well as my account is going to the collections department. I did lose my temper because I thought that this had been sorted. I told the lady to check their records and I want a call back to let me know what is happening as well as an apology. No response back. The messages have carried on this week and I sent an email yesterday as well as today to find out what is going on as I need to pay my account but cannot do that until the previous payment has been sorted. This is very poor customer service and I will be making a plan to close my account. Rather chase the problem payers and not the innocent people that do pay.