The Do Not Disturb (DND) feature allows you to set your phone to forward calls without ringing the phone. Enable the DND service with the DND soft key on Cisco Unified IP phones that support soft keys. When DND is enabled, inbound calls do not ring the phone, but they do provide visual alerts and call information. They can also be answered if desired
One thing to note is that DND is a part of IPMA in Cisco CallManager 4.x and 5.x. (In Cisco CallManager 6.x, the DND feature can be used without IPMA.)
for ur case there are several workarounds to use DND in CallManager 4.x and 5.x without IPMA:
1: In order to use the DND feature without the IPMA service, enable Call Forward All to your voice mail. This forwards all calls to your voice mail without the phone ringing.
2: Similar to the first workaround, create Speed Dial to Voice Mail, and label it DND on the phone. Whenever you need to switch to the DND mode, press CFwdAll and then the DND speed dial button.
3: Another simple workaround to implement DND is to use a silent ring tone for the phone.
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