Worst service ever!!!!!

Worst Service ever!!!! How is it that when you want to buy the product the service is great but then the after sales service is so ****. I was promised a discount on my upgrade for the first 3 months, because i complained about the service but this has not materialized. When i call into the call centre im told i cannot be transferred through to the billing department and they will give me a call back but this does not happen. No one calls me back