VoLTE is not very well received standard, that will probably not last long anyways. It's also a cheap-o-way for network operators to offload traffic off the 2G/3G nets for voice.
In other words, VoLTE is not a priority to anyone, but only those who actually use it. Don't expect support in any ROM for that.
Oh and by the way, Moto X ghost supports VoLTE since KK, but only the standard defined in the 4G networks definition and not the cheap, abomination that is deployed in countries like India. The real deal supports 3G -> VoLTE and reverse, while those badly deployed networks support only VoLTE<->VoLTE calls only.