LTE was originally designed as a data-only upgrade to 3G. It became the full on successor to 3G, but launched with voice spec left as TBD. It eventually got def

LTE was originally designed as a data-only upgrade to 3G. It became the full on successor to 3G, but launched with voice spec left as TBD. It eventually got defined, but ended up being a cursed nonstandard SIP-VoIP software thing, and so a lot of carriers and networks implement whitelists for VoLTE phones to manage anxieties around it.