So, let me ask you something. Does your wife have WiFi calling enabled on her phone? Turns out there is a bug with this and if you have it on it kills sprints HD voice. HD voice will not work when a call is sent over WiFi and this is "by design". However, when on a cell based call HD voice should still work even with wifi calling enabled, yet currently, due to this bug that is not the case. I know this from first hand experience. I turned on wifi calling when I got my phone and I could not understand a word my wife was saying when I called her phone (iPhone 6s plus to IPhone 6s Plus) she also reported that I sounded horrible and couldn't understand me. So, I searched for the problem and found out about this bug. To fix it so the following.
1. Settings - Phone - disable wifi calling you also need to delete he emergency address that was entered.
2. Then go to phone and dial ##update# to refresh your services.
3. Make a test call to another sprint phone and after a second or 2 HD voice should kick in and be very clear.