Voice garbled on phone calls before and after repair

Since receiving the phone, outgoing calls have been garbled to the person on the other end of the call. The phone was given to me by my girlfriend and she said that she's never had this issue with the phone.

I did a little research, and it appears that the iPhone 6 is notorious for having noise cancellation issues and most of the threads online suggest that I should replace it at an Apple store - which isn't an option.

I replaced the front-facing camera/secondary microphone assembly, which seemed to help, but the calls are still fairly muffled/garbled on the other end. I double checked all connections and placement to ensure that everything was done correctly but it seems as though the phone still exhibits the same signs despite of the repair.

Ironically enough, the best call quality comes from toggling "noise cancellation" to "off" in the settings, and then covering the mesh port on the back of the phone during the call.

What I've done so far:

- Checked all connections

- Replaced microphone/camera assembly

- Toggled noise cancellation and made outgoing calls

- Hard reset

- Soft Reset

- Reset all settings

I'm considering just placing a small piece of tape over the mesh port temporarily since I've had this phone apart several times and each disassembly hasn't yielded any worthwhile results. Any suggestions would be greatly appreciated!