I have had an account with them for 3 yrs

I have had an account with them for 3 yrs. Fibre package. For the past few months my invoice has been inconsistent. I have had to pay more than initial installment. Trying to contact them to query is a nightmare, when you do eventually get through the consultant is not able to assist, as you get told that they assume. Assume what? Long story short apparently my premium went up and accounts department failed to update my invoice. No communication what so ever and now I am liable to pay a bill that is not my fault. I am furious at their service and furious at the fact that a well established telecommunications company could render such poor services.