VoLTE is all-IP, but it is not an "OTT" technology. It's "carrier SIP for your phone number." At best, it could deliver higher voice quality between compatible endpoints. But it hasn't taken off because the advantages are mostly theoretical.
Being a carrier voice product, it probably will not have any advantages over other carrier voice products, e.g. international calls will still be the same price, etc.
Being a carrier voice product, it probably will not have any advantages over other carrier voice products, e.g. international calls will still be the same price, etc.