fixed phone but caused more problems

I took my phone to get repaired because it could not dial numbers. Vodacom in Alberton the repair store were able to fix it but ever since my phone was repaired there it can seem to pick up network. I want to back and was told that they can't not tell me what is wrong with my phone. I even called the call centre and they also can not assist me. I am upset because what is the point of having a phone that I can not use. The worse part is that my phone is on contract and I am paying for a phone that I am not using.