You overestimate the difficulty. First of all, this is not two appearances of the same phone in any way, shape or form. Many companies already offer ringing two or more lines at once for a call. It's not just easy, it's a solved matter. I can do it myself, and ring both my cell phones at once in this fashion with no difficulty, and have done this. The only thing that actually stands in the way is accounting -- to do this I have to have two cell accounts, with two monthly fees, or two phones on the same account with a high monthly fee.

The other technical items are minor. You could implement this in short order by giving the other phone a 'number' that can't be called except by the forking agent which rings multiple phones, and map its caller-id on any outgoing calls and you would be done. However, that's a kludge to get it done in a a short time, doing it right is really not that much work.

Once again this is not only doable, I have already done it with my home computer. It's mainly a question of billing and caller-id manipulations.