Over billed

I've got an account since Jan. 2018. And last month I've realize that I've being over billed. First it was a struggle to any one of the call centre. The one day I've been on hold for 15 minutes and then still no operator available. I've send emails and sms'e..no communication from their side. Yesterday I've got hold of a person. He told me there's an "add on" on my account. He does not know how it happen but he will reverse it. He could not send me a new bill or any confirmation only a case number. He just said that on the 20th of this month they will subtrack the correct amount.