Hi,
Actually, you can reject the call of caller by pressing the key and the caller will receive a busy signal.
Alternatives to busy signal and dial tone are the redirect function on a busy signal, automatic callback, or the use of a voice mailbox.
More details:
http://blog.colima.de/en/2012/04/busyonbusy-bei-lync-was-ist-das/
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Kent Huang
TechNet Community Support
