Promises not delivered

I have had a nasty experience with Belgacom services. The initial contract which I signed up with them has not been respected. And to my surprise they invoiced me some entries which were always GRTAUIT, good that I looked and now they send revised invoice but again highlighting HIDDEN Charges, which were not transparent upfront. I have wasted lotsa time by communicating with them but of NO usage. Can someone guide me how can I lodge a complaint, etc. so that I get what I deserve. Thanks much !!