Xiaomi India - Service Center Not helping properly (Bad Behaviour)

bought a xiamoi product (Redmi y*) which had a issue while taking calls (tearing noise while receiving and having any calls) . We went to xiamoi service center to get this addressed, but the technician were not able to catch issue , later we demonstrated and reproduced the issue , after this we were asked to wait for ** -** mins and as per the tech lead they changed some receiver but the issue was still occurring and the same was excepted by the lead but he was said that he cannot help more and i need to sign on a paper stating issue is fixed. After long conversation i denied signing on the paper . i requested for a receipt with comments that issue still exists , but he denied and started behaving aggressively. later he said i dont need any kind of signature. Whom should we now contact so that i get a defect free phone.