What does it mean when the mobile phone notification bar displays this HD? Not usually available

The notification bar HD represents 4G high-definition voice calls (Volte).

The reason why it is not available now is because the operator has enabled high-definition voice calling, and the mobile phone also supports it, so it is displayed in the status bar.

HD voice calls can make calls clearer and connect faster. It also supports video calls and the network will not be interrupted during the call.

VoLTE is an IMS-based voice service. IMS has become the core network standard architecture in the all-IP era because it supports multiple accesses and rich multimedia services. After maturing in the past few years, IMS has now crossed the rift and become the mainstream choice for VoBB and PSTN network reform in the fixed voice field. It has also been identified as the standard architecture for mobile voice by 3GPP and GSMA. VoLTE is Voice over LTE. It is an IP data transmission technology that does not require a 2G/3G network. All services are carried on the 4G network, which can realize the unification of data and voice services under the same network. In other words, 4G networks not only provide high-speed data services, but also provide high-quality audio and video calls, and the latter requires VoLTE technology.