Muffled sound when on calls

I bought my iPhone 12 mini 10 days ago, and started using it about 3-4 days ago. I noticed that the call sound has been terrible, sounding very muffled when i'm listening to other people on call. I tried all the software tips (from my own research as well as from using Apple online support) with no success. I also brought it to the apple store and to a certified apple service provider, who were also unable to fix the issue but they said that the device passed all their tests and diagnostics. I still don't understand how such a poor sound quality could pass these tests to be frank, but i'm just out of ideas. I also tried calling on whatsapp and the quality is actually better (which makes me think it may be a software issue). The person on the other end of the call always sounds like they're on speaker for some reason.

Another option i was thinking of was that maybe it was from a poor sim card (my previous phone had fallen into the sea, but otherwise it was working well). If anyone has any ideas or is facing the same issue, please let me know, I am really unhappy with the service or the product provided by Apple and they just wanted to get rid of me as soon as possible while i was in store. If this does not get fixed, I will probably ask for a refund as I cannot use this poor call quality for my work.

Please help if anyone has any ideas or any fixes!!