Regarding HD Voice, there is no mention, so I have my doubts.
Please understand, that I have taken the freedom to remove the link at the end of your posting, as it was not connected to your posting and therefore might be considered spam.
Everyone interested can still look it up.
EDIT: I was a bit slow and therefore removed all the double posting.
Overall it looks, like the FP 3+ is somewhat lacking for your needs.
And you should always keep in mind, that every repair issue in the US needs to be serviced here in Europe. So you have to buy and ship spareparts or the phone. And coming by a spare battery might not be that easy either, since they are considered hazardous goods when not shiped inside a device but on their own.
The FP3(+) has HD Voice. It is pretty much standard in all smartphones since many years, no need to worry about
I have not done Non-HD-Calls anymore since years (*).
To be more technically: G.722 (the codec for HD Voice) started in mobile networks around 2010 / 2011. At least here in Germany also the “interconnect” (phone calls between different carriers) are all done in HD Voice.
Of course it depends on the carrier. But I guess that all carriers in the US use HD Voice nowadays, too.
Not to be confused with HD Voice Plus (EVS). As far as I know this is not possible with the FP3(+).
(*) Speaking about mobile calls. Of course, if you are calling an old landline number which is working in POTS mode since decades, this will be no HD Voice.