@atarilinux I'll start by saying that VoLTE, while being used daily by some, sounds like it should still be considered in beta across all devices.
You're right that Volla and Fairphone don't sell outside the EU, at least for now. @AppLee recently mentioned that Volla is looking into official network support in North America, so who knows, maybe they'll sell phones here one day.
Meanwhile, there is Murena, a third party vendor, that will sell Fairphones to NA, or third party shipping services, which I know little about, but will ship here for a markup.
There are also Fairphone 4 phones on eBay from a vendor called NeverMSRP that I considered for a bit. They seem to have good reviews.
But there are also people using Xiaomi phones with VoLTE, including in USA I believe. Poco X3, Redmi Note 9 Pro, and maybe others. The ports for these phones seem to be solid and supported. I found those phones hard to find domestically, but careful searches on eBay yielded international vendors who'd sell to Canada, though tariff stuff may be complicated in USA.
And then Brax3 phone and Furiphone FLX1 are both making ports of UT, though we're awaiting details. Both play well with USA networks though.
Hard time to make a decision, with VoLTE and 24.04 still in progress, but we can at least help each other be aware of the options and make the most informed decision possible.