I always how VoLTE can be a phone specific thing for some operators only! Sounds strange.
According to Idea Cellular’s Rajat Mukarji there is no standard in VoLTE so each operator choose their own
We can’t afford to run our network at current IUC rates: Idea Cellular’s Rajat Mukarji - ET TelecomWe're experimenting with systems right now because these are not very standardized things right now. Particularly, for example VoLTE, the standards have not yet been finalized for it. There are different types of codex, there are types of compression devices and even handsets have to match that. So it's a complex task right now, but we're testing out things.
I think he wants to say Codec and it's the handset manufacturer duty to push updates in open market like in India where carrier does not control device software updates. And with very low profit margin mobile manufacturer does not bother about pushing thoses software updates to old phones which are already sold, so only popular old models gets those codec and then support VoLTE. All new phone will come built in support though