Today the Microsoft Lync Team announced through Twitter that the Lync Push Notification Server will be down for service maintenance until the 8th of October 2012.
"NOTICE: Push Notification Service is down until Monday Oct 8 2012 due to service upgrades and maintenance. Spread the word! "
The Push notification service is used by Lync Server to send Push messages to Windows Phone and iOS devices. As long as the service is down, iOS and Windows Phone will not receive any Push notifications.