Lower price only for new customers? What about existing customers?

We have been using Vox as our service provider for two years. Their pricing for 10mbps, 300gb data, 1gb mobile data, superphone, and wi-fi router went down by about R100 almost a year ago. As an existing customer we were not given the benefit of the lower price and have been paying the higher price all along. The only reason we knew the price had gone down was that we were asked to do a price comparison of all service providers in our area, and then were horrified to discover that we were being over charged. We've not been able to get any management intervention, they do not phone back, whilst the poor young salesman is embarrassed to say the least. At the every least I expect a credit on my account prior to cancelling Vox as my service provider. It is correct that existing customers do not automatically derive the lower price - surely this is tantamount to *****? Or is such customer service now the norm in our ******* riddled country?