About push notifications on IOS

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Good day! Please help. I have deployed flexisip server with basic configuration. We use tls. The linphone app is installed on Android and iOS from the market. The problem occurs on iOS - when the application is minimized, calls are not received. What should I do to make the push work? Doesn't the linphone app have a connection with the apple certificate authority, to wake up the app via apn ? Or is it mandatory to sign flexisip server with apple ? I apologize for the question, I'm not really a developer.