Both voice and video calls depend on how we stream media between the two clients which are connected to each other. So, there must be something that can do the work of media streaming from one client to another client.
For media streaming, we need to know about WebRTC.
WebRTC is a free, open project that provides browsers and mobile applications with Real-Time Communications (RTC) capabilities via simple APIs. The WebRTC components have been optimized to best serve this purpose.