In early 2013 ABI forecasted 785M LTE subscribers by 2017 (here).
"The extensive presence of LTE networks is also stimulating the deployment of VoLTE. By the end of 2014, there are expected to be 59.6 million VoLTE subscriptions in place, and it is expected that nearly 56% of LTE-related cellular subscriptions will be using VoLTE services by the end of 2019 .. Mobile operators have continued to invest significantly in LTE network, which will cover around 64% of the world’s population by 2019"
See "Expanding LTE Coverage is Priming VoLTE Adoption and Mobile Devices Shipments" - here.
See "Expanding LTE Coverage is Priming VoLTE Adoption and Mobile Devices Shipments" - here.