The problem with your situation is it assumes that everyone is there to answer a call simultaneously (which is probably not true), and that the system will ring every phone until such time as it is answered, and automatically connect everyone to a group conference.
It seems that what you really want is a dial in conference bridge service, coupled with a "group page" (Like belt pagers or cell phone text messaging). Use the group page to alert others to call.
There really isn't a feature that would allow a call to continue to ring at other desks after it's been answered at a first location. (How would that sound? Would you get an answer and still hear a phone ringing?) Frankly, I don't remember such a feature on Nortels either. (Don't know about Seimens.)
The coverage answer group is probably closest, but if a person activates send all calls on their phone, they remove themselves from the group, and like you said, only one person can answer.
You might just want a bridged appearance on every phone that everyone can pickup if they hear the ring. It won't include the cell users, but they can be added to a conference after.
There's a system made by a company I used to work for that has this feature, but it's a pricy system for just an emergency situation. You're better off going with pagers/cellphone text messages and a conference bridge company that let's you do dial in conferences without scheduling them.
Carpe dialem! (Seize the line!)
