Overcharge on Contact all off a sudden

ACC***(C.Pienaar) After many years and many contracts, out of the blue, I am debited over n 1000 instead of contracted 638. Fine, get in touch with customer services by mail. Promised 24 hr response. Good grief, a week later we are still debating account and cell nr. This after all the basic info is in the subject line. Truly disappointed, it's like dealing with a Govt Dept. Not so much the extra money, but the principle and NO help... Geez... Anyway only realised today I can reverse the debit, then I'll be in arrears, and then I'll get called