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.
To resolve this issue, install the following service pack:
2817430 Description of Microsoft Office 2013 Service Pack 1 (SP1)