network connection in iPhone disappear frequent

I have bought an iPhone 6 from Tata Chorme, Chennai, India 8 month ago. I was using Airtel SIM and found fluctuation in network connectivity. Network tower symbol and operator name was getting disappeared frequently. I thought it was the issue of Network operator and changed the network connection to Vodafone. But I am finding same issue in Vodafone connection also. I have used same SIM in other iPhones and not found any fluctuation in network connectivity. I have approached Apple iCare for the support, but they are not ready to accept the complaint. Apple iCare is saying the issue is with network operator and not with Apple device. In addition to that I am finding may other issues with my phone and the same are as follows.

1. When other try to call my number, most of the time they are getting switched off message
2. Most of the time unable to send message and the same will be returned
3. If I play music in Youtube, even if I minimise Youtube app, I am able to hear music.
4. While playing Youtube music, if I get a call both Call voice and Music voice are audible sometimes.

iCare team has reinstalled the OS many times, but the issue still persist. The phone is still under warranty around 125 days still left. Request Apple team’s support to resolve the issue. Please note that I am a regular iPhone user and never phase these kind of issues in my other iPhones and Mac Devices.