Amazon India - Battery backup and sound quality is very poor

The last date of return for my MI-a* black mobile was *th October,****. Due to poor sound quality and battery backup I decided to return it and choose return option. And then a Blanco test occurred. After that I had a interaction with Mr. Vishal, a service provider through Amazon on-line help desk. He assured me that my return file is accepted and after using that worsted phone for ** hours I have to call again ,then return and refund process will done. But when I call them today they simply denied and ask me that I didn't make any call. But then and there Mr. Neeraj transferred the call to his senior person named Mr. Gaurav. He asked me that he will replace it. But I didn't agreed. I had a urgent meeting in that time so I told him that I'll call on tonight. He assured me customer service provider will help me. But when I make the call at tonight the service provider simply denied and ask me that they had no data about my complaints. So I felt very insulted. Please take necessary action.