You may want to think about shared line as an option, but it does ring at the same time.
Cisco Unified CallManager Assistant is a feature that is designed to do what you are looking for but it may require installing a client application and some configuration.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/5_1_3/ccmfeat/fsipma.html
The other simpler way is to configure the same using CFNA and CFB. If 1000 is the number for the Manager, set it to forward to 2000 which is the secretary and on 2000 set the Call forward no answer and busy to forward to Boss extension say 3000, set the ring no answer timer to 8 - 10 seconds for 2 rings. You can also then set 3000 to CFNA and CFB to 2000 or to a VM box.
:JoeL
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