I have an iphone user that has the app installed and even though the app is running in the background, when you call his extension, the app does not ring and pop up. I have made sure that the push server is whitelisted but I am not sure if the new app even uses push.
Take a look in Reports | PBX Reports | PJSIP Endpoints. Check that the extension in question is showing a contact, it should be showing a registration by the push server in addition to or instead of the phone itself.
Which app is the iPhone user using?