VoLTE is connecting stack that uses SIP tech to establish voice calls using IP Multimedia Subsystem (IMS) functionality of deices and IMS infrastructure run by Carrier over LTE network.
One phone side, it’s softwae/blob component exploiting LTE/IMS cabalitiy of the baseband processor.
IMS uses TCP-UDP/IP over LTE network to provide voice/video and rich media capabilities to carriers and devices.
Android Framework for IPS
IMS Profile for Voice and SMS Version 9.0