Motorola Moto G5 Plus
The only thing I can think of, is certain carriers are different. My v20 (albeit broken now) worked on VOLTE with Sprint, being it was a sprint branded device. At present, G5P does not support this with Sprint, but a buddy has one on ATT and his volte works fine.
I think it's solely dependant on the carrier-- however, for me, I live in an area that has GREAT reception thru sprint, not to mention, having it be a...