I was trying to run a conference call yesterday from my softphone and kept getting the message "Conference cannot be completed." Thought it was a quirk with the softphone, so tried again today from my deskphone. Same message. Jumped into CCA, and noticed that there is a note under Configure>Telephone>Conference"
"Note: Conferencing disabled - detected external configuraiton"
I'm not sure how long this has been disabled, but I'm guessing that it hasn't been longer than a week or two. Could this occur if I have setup SIP trunking in the meantime? Anything else that could turn off conferencing?