FP2 doesn’t support any 4G bands used in the US, so it’s not really suitable as a phone for daily use there. And as others have mentioned before: There are no known plans for officially selling or supporting Fairphones outside of Europe. Don’t expect that anytime soon. FP2 and FP3 are not even FCC certified.
This being said: If you manage to import a FP3 yourself, it should work. More or less. Unlike the FP2, it supports many frequency bands used in the US. You can easily check how well it might work with your carrier. Let’s take T-Mobile US as an example:
- Check what frequencies / bands they use for what services, e. g. on Wikipedia.
As you can see, they use bands 2, 4, 5, 12, 66 and 71 for 4G (LTE), 2 and 4 for 3G (UMTS) and 2 for 2G (GSM). Also read the notes. Things change and you will want to use your phone for a few years. - Match these to the frequency bands supported by FP3 (listed in the Network section of the tech specs): For 4G, bands 2, 4 and 5 match; for 3G and 2G all bands match.
So it should work in all areas except those which have 4G coverage on bands 12, 66 or 71 only. 12 and 71 use lower frequencies, so these will most likely be used in rural areas.
You can do the same with any other operator. But there’s no guarantee that it works. Be bold and try for yourself!