Maybe this is a trivial question, however it is not yet clear to me if the server side is impacted when PushKit frameworks will be applied in a VoIP iOS app.
The SIP server for example sends an INVITE to my app, the notification is received in the app and the call is established.
Is the server impacted when PushKit.framework is used? Who is sending the pushkit event? The SIP server or Apple? Is this similar to APNS?
It is exactly like APNS as far as the server is concerned. It just comes with added benefits for the client (the iOS app).
The client registers with the Apple servers and gets a token in return. The app registers the token with your server. Your SIP server can use the token to send the notification with the appropriate payload. That is sent to APNS which then forwards it to the device.