On my unlocked-variant Samsung S8+, you had to flash the carrier firmware through Odin to get the more advanced features like Wifi-Calling and Advanced Calling but you couldn't get most of them. For TMob, I flashed the T-mobile firmware and was able to get Wifi-calling and AC but could not use the built-in DIGITS support that the phone had. For VZW, i flashed the VZW firmware to obtain VoLTE and AC, but still could not use the video calling features. It really is up to the carriers and how the whitelist the IMEIs to enable all the carrier-specific features. Either way, the stock unlocked firmware didn't have access to certain features that the carrier-variants had.
Based on everyone's experiences, I honestly feel like the carrier-variants will have their IMEIs whitelisted with the proper firmware to support the features. The unlocked variants would not and you may have to go the root of flashing carrier-firmware to get some but potentially not all of the carrier-specific features. The phones are all the same and "compatible" with each network, its the firmware and whitelisting that will end up being differen