When the APNS server is told by the viber server that something is happening for specific device token, the APNS server will contact that specific phone and send a message to the phone. The connection between the iPhone and the APNS server is built-in to the iOS platform and it happens automatically on a regular basis. It will handle notifications for all of them. The APNS server keeps track of all apps the user has on his/her phone. When the user receives a call, the viber server contacts the APNS server, which in turn alerts the user. The server will connect it to a specific user. The app connects to the application server (for example Viber's server) and sends the device token also to this server. This token is a serial number that helps the Apple Server to keep track of the phone. If the user accepts, the app registers to the APNS server (hosted by Apple) and registers a 'device token'. When the app is installed, the user is asked if the app should be allowed to send push messages to the phone. In short (and simplified), it works like this This is a service provided by Apple that helps applications to alert users when something happens. For iOS, these apps use APNS (Apple Push Notification Service).

Viber vs whatsapp