android - How to receive calls/messages real-time when the application is closed

Question

I am developing a VoIP application with SIP as the signaling protocol. I have implemented the messaging and calling functions and they work when the app is in the foreground. To make the app listen to the call and message receiving when it is closed, I created a foreground notification so that the app would always be in the foreground.

The problem is that the above method consumes a lot of power which is unacceptable. I have also noted that famous VoIP applications like WhatsApp and Signal are not using foreground notifications but, they manage to inform us about incoming calls/messages no matter the app is closed. I wonder how to achieve the same functionality. Could anybody please point me in the right direction?

Answer 1

This kind of App should use Push Notifications (reference: https://firebase.google.com/docs/cloud-messaging) that use ONE SINGLE socket connection for the whole system: each App should register itself for receiving Push, then when the Server triggers (many languages are supported by FCM with examples/tutorials) the Calling Event then a special "message" is sent to the App (providing some information about IP or whatever you want) that gets those information and acts in the proper way.

Unfortunately FCM, after few thousand notification per month, requires payment. There are other similar online service, but those services requires a fee because they should handle many users connected at the same time and it requires hardwar and bandwidth.