Apple appears to be gearing up for high-definition voice-over-LTE (HD VoLTE) support in future mobile products The company is currently advertising for a cellular systems protocol engineer, calling for experience with “VoLTE” and “IMS.” Other positions are calling for experience with the Session Initiation Protocol (SIP) protocol. VoLTE uses SIP to initiate the 4G calls, which are transmitted in data packets, with the IP Multimedia Subsystem (IMS) in the core network controlling the call session. There’s a useful primer on how this all works here.