How to fix poor voice quality on iPhone X

We recently upgraded my wife from an iPhone 6S Plus to iPhone X (256 GB). I have noticed that her voice sounds very tiny and compressed (no depth or base to the sound) when listening to her on voice calls. It can be hard to hear her. We first noticed this on calls between her iPhone X and my iPhone 8 on same carrier (Verizon). The voice quality is just not on par with the iPhone 6S Plus or my iPhone 8. This is disappointing considering how expensive the iPhone X is.

I have already swapped the iPhone X for a new one at Verizon (thank you Verizon for making this easy). This did not help, so this leads me to believe there is a larger issue either with the iPhone X hardware, compression algorithm used with iPhone X, or with call quality when the phone is used on Verizon. However, with respect to carrier, I see that other folks are reporting issues with other carriers as well (e.g. AT&T).

Other diagnostic steps I have taken:

1. Checked for same poor voice quality on call between iPhone X and other phones--iPhone 6 and landlines.
2. Verified cell signal is decent--at least two bars.
3. Made sure Wi-Fi calling is off in settings.
4. Reset the phone and completed set up again.
5. Applied latest iOS updates (11.3).
6. Looked for any obvious obstruction around mics.

The phone is in an Otterbox case (Symmetry Clear Series), which covers the back and sides. It is not a folio, which some have reported problems with. I believe the mics are all unobstructed, but I can't find confirmation of their exact location. Presume bottom and rear near cameras.

I suppose the next step is to take it into Apple, but our nearest store is a three hour round trip drive. Not looking forward to that time sink.

This is disappointing considering how expensive the phone is.

Any suggestions for additional diagnostic steps are appreciated.