Bad service/inferior parts

About a month ago I was at the cape town branch to have my phone screen replaced under warranty I also spent money on a new casing not covered by warranty and requested a back cover(no stock)so I do not have a problem paying for repairs or extras.I noticed that the new screens came with a screengaurd(good initiative)and commented to the technician repairing my device who rep**** it comes with the new screen my previous did not.On using my home screen button a crack ran throughout my screen.I contacted cape town branch on the weekend of the 24/ 7/15 who advised me to bring in my device on the 27/7/15.I did this and spoke to the manager who took photos and said he would contact me same day.This did not happen and I spent the rest of the week calling and leaving messages. I also contacted the headoffice repeatedly where nobody even bothered to answer the phone each time i tried.On the 30/7/15 the manager calls me to tell me I have to pay to replace my screen.Why should I if they could be using an inferior/defective part.I feel that this is unacceptable and the matter will be taken up with a higher authority.