The Do Not Disturb (DND) button works with the Cisco IP Manager Assistant (IPMA) service, and it is not active for use outside of IPMA.
Even though DND is not supported without IPMA, these alternatives which can simulate DND without IPMA:
Use the CFwdALL, forwards calls to another extension or voice mail, or the Immediate Divert (iDivert) buttons. The iDivert feature immediately diverts a call to a voice messaging system. When the call is diverted, the line becomes available to make or receive new calls. Refer to Immediate Divert for more information.
Use a multiline phone such as a 7940 or 7960, and define a speed dial that is an extension to voice mail or another destination of choice. Call the speed dial DND. When a DND is needed, press the CFwdALL button. Then, press the speed dial button called DND. When a call then comes to the phone, the phone does not ring, and the caller is sent to the DND speed dial destination. Two buttons must be pressed rather than one, but it accomplishes the same objective.
Alternatively, a silent ringtone can also be used when a user does not want to be disturbed, and after a certain number of rings calls are forwarded to the voice mail of the respective users.
There is no option to set a phone within a hunt group to DND if the user is not available. But, extension mobility can be used in order to allow the user to log in or log off. This feature is introduced in the CallManager 6.x onwards and it is not dependent on the IPMA. Instead, there is a default softkey DND on all the IP phones.