Assume that a user (user A) makes an audio call to another user (user B) by using a client for Microsoft Lync. Then, user B immediately starts to speak after they answer the call by using a Lync-based device that is paired with a Lync 2013 client. Or, user B answers the call by using Lync 2013 in an environment that has a long network delay between Lync Server 2013 and Lync 2013, such as in a Lync Online environment. In this situation, user A may miss the first few words that user B speaks.
This issue occurs because the media flow starts after a callee receives an ACK message in Lync 2013. However, when the callee sends a "200 OK" response to accept the call, a network delay may occur before the callee receives an ACK response. Therefore, the media flow does not start immediately when the callee answers the call.