[Samsung Galaxy S9  Questions & Answers] mobileanimal:
Hey I'm thinking about buying a G960FD as well for use in the US. I have G930FD's and G950FD's currently. The F models don't have LTE bands 30 (AT&T) or 71 (T-Mobile) - frequencycheck.com is a great site to look up phone/carrier compatibility. The lack of these frequencies in the radio is not a deal breaker because these are just for extra capacity at this point, though band 71 with T-Mobile offers additional outdoor range being low frequency. You will need to root to in order to get VoLTE in the US, and this will be necessary for T-Mobile since T-Mobile only has 4G coverage and some crappy 2G coverage outside of large cities. So without 3G coverage calls WILL fail outside of large cities if you don't root to enable VoLTE since the phone will not be able to place a call on 4G LTE bands without VoLTE. AT&T doesn't typically allow non AT&T unlocked phone IMEI's to use VoLTE, and AT&T will be shutting down 3g in a few years. At that point, their IMEI whitelisting may go away and they'll start allowing an phone to use VoLTE, otherwise they'll start losing customers. If they don't change their attitude though, your phone will also not work on AT&T in a couple years (they don't have 2G enabled anywhere anymore). Verizon is also shutting down their 3G network in a couple of years.

I actually came searching XDA to see if anyone had a SM-G960F or SM-G965F working with VoLTE (and specifically dual VoLTE on the FD since it's supported) on any US carriers. I haven't come across anyone yet. On the Exynos S7 and S8, if you don't need WiFI calling, it's actually pretty easy to get VoLTE working without having to install an bunch of crap. There's just 4 files you need to edit or overwrite in /system/csc and one additional VoLTE settings file you have to place there as well. I'm thinking it would be the same with the Exynos S9. I just need someone with an S9 to confirm. Since you need WiFi calling, it's going to be a bit more work. You can look at the CSC selection tool easily found by searching here on XDA for the Exynos S7/S8 to see what you'll need to get WiFi calling to work (it's not that bad to go through the code and files in the zip installer). You may end up having to extract the US Qualcomm S9 T-Mobile firmware to get the required files out of it, or you might even be able to get away with simply flashing edited portions of the S8 CSC selection tool files to your G960F to get VoLTE and T-Mobile WiFi calling to work. (If you flash the CSC selection tool to directly to the stock ROM, it will break a lot of things due to the permissions XML file). I would plan on spending a few weekends to get VoLTE and WiFi calling working on it though.

If anyone has any additional experience, please chime in here!!

Also, one thing to pay attention to whenever you flash stock firmware using ODIN is Samsung's stupid e-fuses that prevent you from downgrading to a different bootloader level which will lock you out of downgrading stock firmware. It's the first number in the firmware descriptor. For example, if you blindly flash a U3 or S3 firmware, you can NEVER go back to a U2 or S2 firmware outside of a custom ROM. And even with a custom ROM, you may end up with compatibility issues with the new bootloader.

Another thing is when you root Samsung phones, it trips another permanent e-fuse that not only voids your warranty, but prevents apps like Knox and Samsung Pay from working ever again on the device.

To answer your questions:

The FD is just dual SIM - no other advantages. There's actually a hack for the Exynos S7's that will convert them to dual SIM since it's the same hardware. I'm not sure if it's like that with the Exynos S9.

Yes, the Exynos international versions have an unlocked bootloader (just have to go to enable developer options in the OS then go into developer options and enable OEM unlocking)

Yes, having an unlocked bootloader means there will be MUCH more custom ROMS and development out there.