VoLTE (Voice over LTE)
Updated
VoLTE carries phone calls as IP packets over 4G, using the IP Multimedia Subsystem (IMS) of the operator. The minimum profile that devices and networks implement is defined by GSMA IR.92.
In practice
Devices that make calls, such as alarm panels, lift phones and emergency buttons, keep voice after the 2G and 3G shutdowns only if their module supports VoLTE and the network accepts it, which is not guaranteed in roaming.