I want to set the expectation here- devices in the USA must be certified to register to IMS and use VoLTE. This is why VoLTE won’t work on the 10 iii under Android on a USA carrier right now. It could change down the line or perhaps a workaround found. I do know of “ways” to make it work. They are not technically violations of any specific law in the USA, but they are highly likely to garner users’ negative attention from the digital authorities here- namely the NSA.
One addendum- if anyone finds a carrier in the USA that does consider the 10 ii or 10 iii VoLTE certified, please let me know. I haven’t found any, but that’s not to say there are none or that the details I have are exclusively correct.