Consumer Redressal

I had gifted a HTC Desire C Smartphone to my elder sister on 13/11/2013. The bill is in the name of Shaista Riyaz. Initially the handset used to work fine. But as days passed, there was a problem that when on a call, the other persons voice becomes blank for 15-20 seconds and again it becomes normal. Facing this issue, i had given the handset to the Shopkeeper from where i bought it for repair. The shopkeeper in turn had sent the same to the respective service center. I got my handset back after a week or so, stating that the issue had been resolved. But i was still facing the same issue, so again i gave the handset for repair via the same procedure. The service center guy said that he will be replacing the motherboard and updating the software which will resolve the issue. This happened 3 times and still the handset is not working properly. It has caused me very much mental agony and waste of time and energy. Kindly help me resolving the issue