VoLTE
VoLTE stands for Voice over LTE, i.e. 4G calling. 
You can automatically benefit from VoLTE technology if you have a compatible smartphone.

Calls via WiFi
Do you sometimes experience weaker mobile (4G/5G) network coverage, but are connected to Wi-Fi? Good news: you can also make calls via the Wi-Fi network (Voice over WiFi) when the mobile network coverage at your home is insufficient. Therefore, having good Wi-Fi coverage is important to ensure optimal call quality over Wi-Fi.