Hi,
You can set the busy trigger to 1 and set call forward busy under your DN to forward the call to a dummy number on CUCM which has CFA set to unity connection. On unity connection, you can create a call handler with the name user_busy and upload greeting "The helpdesk is busy, please wait some time on the line.".
Call Flow would become:
User C >>>> calls >>>> User B (already on call with user A) >>>> Busy trigger set to 1 under dn >>>> Call forward busy set to a dummy number eg 77777 >>> CTI Port (77777) >>> Call forward all to Voice mail >>>> Unity connection >>>> call handler >>> busy greeting
Aseem
(Please rate if useful)