When you try to transfer a call to a mobile device in an audio conferencing provider (ACP) conference in Microsoft Lync 2013 (Microsoft Skype for Business), the call is transferred, but the media stream remains active. Additionally, you receive the following message on the transferred call:
An unknown participant is now joining
This issue occurs because the client endpoint is connected to the Audio Video Multipoint Conferencing Unit (AVMCU) before the call transfer. However, after the transfer, the media session with the AVMCU is not disconnected.
To fix this issue, install the January 12, 2016 update (KB3114502) for Lync 2013 (Skype for Business).
Note Lync 2013 was upgraded to Skype for Business in April 2015.