Xiaomi India — Incoming outgoing calls issue, voice issue

I purchased a new handset redmi 8a dual on 21 may 2020 @ 8000/- from vijaya sales kalkaji, new delhi. From the day one i am getting issue on calls while incoming/outgoing. I visited the kalkaji service station, but appointment given me 30 may 2020.

Service centre not ready to listen, as per come on appointment day. I need to replace this handset but mahesh person service centre denied to listen.
Now what i do, a defective piece of phone i have.
Its my fault if your service center slots of appointments is full.

Please replace my handset. Who will provide du letter after 7 days passed.