not a pbx option.. you can send voice mail to all users at the same time.. but your channels will stay busy for as long as it takes each user to check mail..
group call can call 31 users by dialing one number.. emercency situations... i would not encourage either.. overhead, or some other external alarm is faster.
i did set up a loud bell (claxon) in a plant that took a 4 digit ext and a password to notify a factory area of evac orders...
you could set up a voice intercom, maybe one per dept
john poole
bellsouth business
columbia,sc