Just last night, I upgraded my Cisco 7940 to SIP firmware 8.11. According to documents on Cisco's webpage, http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7960g_7940g/firmware/sip/8_11/english/release/notes/796040sip_811.html#wp1188628, there is a different method for activating Do not Disturb. However, My phone uses the method described in the documentation, which is supposedly the old way. For the longest time, I've been looking for a way to put the DND button on my phone's homescreen, but apparently that isn't possible with the SIP firmware. According to the status page in settings, I have 8.11.
I realize this is in the asterisk forum, but there's not a dedicated hardware forum.