In order to receive incoming call notifications, just like a mobile phone, Push Notifications would provide the alert of an incoming call in the event the application is not running. See: http://developer.apple.com/library/mac/#documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Introduction/Introduction.html
This is a Mac app, not iOS app.
Although I agree that it would be cool, it just doesn’t fit into Telephone’s concept. Telephone is an app that supports any SIP provider. But in order for push notifications to work, there must be custom support for them. This is only possible in a situation when a SIP provider releases its own app or releases an API that Telephone could implement.