Consider the following scenario:
You have some delegates in a Microsoft Lync Server 2013 environment.
You configure the Simultaneous Ring feature to ring all delegates when you or one of the delegates receives an incoming call.
You or one of the delegates answers an incoming call.
In this scenario, the delegates who do not answer the incoming call unexpectedly receive a missed call notification.
This issue occurs because Lync Server 2013 incorrectly generates a missed call notification.
To resolve this issue, install the following cumulative update:
2781550 Description of the cumulative update for Lync Server 2013, Core Components: February 2013