iPhone 7 Hardware gone bad

I had the iPhone 7 with Apple care +. The hardware went bad and whenever calls were made or received there was no audio. Sent phone to Apple and they replaced with a new 7. After 5 months this new phone has the same issue with the hardware. No audio for phone calls. Apple is willing to repair phone but is charging $350.00 because they claim my new phone is not under warranty the original warranty. I was not allowed the option to buy a new warranty for the replacement. I just don't understand why a phone 5 months old has known hardware issues, and they are charging me to fix. When i spoke with the support person on phone who filed my claim she said that Apple is aware if this hardware issue on the iPhone7.