The main two mobile operating systems Android and iOS require that applications should not stay in the foreground and be active listening for calls. Therefore push notifications were developed to allow an application to be activated from a background / terminated state upon receiving a push request. Web push for non-mobile devices is now also gaining traction. So it's something to seriously consider when developing a video or voice application.
However, if the design of your app does not require receiving calls when the app is in background or closed, you do not need to enable push notifications.
One scenario, for example, would be, an app for ordering pizza via an in app to voice application calling a phone number and where the client always originates the call and never receives a call.