Billing incompetence

In November 2014 I negotiated with an ADT salesman to get a sensor replaced in my garden. We agreed a discount and I received a formal quotation, paid, notified ADT of the payment, and had the sensor installed. After the installation ADT sent me a new quotation for the work that was already completed, for a larger amount. I contacted them to tell them that I had already had a lower quotation that had been formally provided by ADT and paid for by me. They said they would sort it out. I got a statement in January 2015 showing that they believed I owed them the difference between the agreed to price and their incorrect later quotation. I also got a phone call in January from a billing person (Tebogo Ramasala) to ask why the money had not been paid. I explained that I had already had this conversation last year and I asked why it had not been resolved. Tebogo asked me to email him all supporting docs and he would make sure this would be resolved once and for all. Two months later I got my statement and the money is still showing as owing. Neither Tebogo nor his manager were prepared to get back to me today to resolve this. I have been a client for 10 years. Shocking service!