VoLTE - Voice over LTE - is the protocol for using the 4G network for phone calls.
2G and 3G networks were primarily phone networks with data added on. So when making a phone call the connection through the the RAN (radio access network) made a decision as to whether this was a phone call or data connection. Phone calls were switched to the PSDN, the nornal phone network.
4G is a n IP network and will deliver the phone call over VoLTE. It is deisgned for the LTE network and makes an efficient use of it. VoLTE does not work over WiFi or any other data connection. In the early days of 4G, VoLTE was not available so the phone would switch back to 2G/3G thus comments that -I cannot make phone calls- were caused becase of the lack of a suitable 2G/3G network. The same fallback wors today. So if a phone with 4G calls a phone with 3G the call will revert to the old 2G/3G network. The quality of the call over 4G is noticably better than over the 2G/3G network. Primarily, but not limited to, the fact that the input sound wave is sampled at twice the rate in 2G/3G.
VoLTE is not the same as VoIP. VoIP is a more general protocol used by Skype, Whatsapp etc. to deliver video or audio calls over any IP / data network. Initially this was synonomous with Voice over WiFi, but actually the Internet can be provided by WiFi, or LTE or even 3G.
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