Terrible call audio quality

I have an odd issue with an iPhone 5c. It's working perfectly, except the audio quality on calls is terrible. The speaker is just staticky and very irritating sounding, and the other end of the call sounds even worse. Constant roaring/hissing and you can barely hear what the person is actually saying.

Now of course my first thought was bad earpiece/mic, but here's the weird thing - the issue persists using earbuds. Furthermore, voicemails sound perfectly fine listening through the normal earpiece.

So it can obviously process audio just fine. And the speaker works just fine. It's something about calling specifically. And this is on Verizon if that matters. I have tried it with LTE on and off and had the owner try it in a totally different area to make sure it wasn't tower-related.

Any ideas?