Incorrect billing

3 years ago I changed my billing date from the first to the 15th, they charged me a bit extra for the extra days and everything went smoothly for a year. Two years ago they staryed smsing me and saying my bill is beyond by a month, I phoned and they investigated and corrected, but the first of the next month same thing and so on for a year. Last year they only take my call, give a reference number and don't come back. If I phone with the reference number they say it doesn't exist and start over with no response and you can never have one direct person to deal with or resolution of your query.