I take it that requirement was written in to ensure Airwave was continued?
Even the ruddy 4G / LTE public networks don't use voice over it as, for some reason[1], a reliable method of fallback / fallforward between that and 3G or even 2G when moving in and out of high-speed coverage is conspicuous by its absence in the standards.
[1] "Chucked on the too-hard pile" looks favourite.