Don't only blame asus
Volte requires paying certification per carrier per phone unlike 3g, and monopolistic carriers (read: most carriers) will overcharge oem manifacturers, with some leaks stating as much as a million $ to get certification.
Given that, it may be more prudent finacially-wise for them to only release 4g data + 3g calls.
Without knowing the exact oem charges it would be impossible to be certain. Most certainly such info would be under strictest NDA if only to shift PR blame onto OEM rather than carriers. But if it were reasonably priced for asus they would have included it.
The mods here r probably either too low ranked to know or under NDA. Don't expect much from thrm
So, unless we get lucky with a leak, won't know.