Muffled voices on iPhone 8 plus

I bought my phone brand new. I am very frustrated because when I talk to another person on their cell phone whether it is an iPhone or not, I sound muffled to them and they sound muffled to me. I would like to upgrade and was told that I can't until the phone is paid in full. This is wrong. I have also heard that this is a common problem with this phone. I have been a loyal apple customer for a long time and I am frustrated by the lack of help from Apple to resolve this. They have tested the phone and claim that there are no issues with phone. I disagree. Has anyone had this issue? Thank you!