Assume that you call a user in Microsoft Lync for Mac 2011, and the user is already in a different call. In this situation, you receive a "Busy Here" message. However, you still hear the ringing sound. The ringing stops only after you close the call window.
After you receive the SIP 486 "Busy Here" message, Lync for Mac 2011 should stop playing the ringing sound, start to play a busy signal, and then stop the call.