Allow two extensions to answer same call

my boss is looking for an interesting setup between him and his assistant.

He would like an incoming call to ring both his and his assistant’s extensions. If he should answer the call ringing should stop for assistant. If his assistant should answer then his extension should keep on ringing and if he picks up he should be placed on the line with both his assistant and the caller.

I had been thinking perhaps I could have an incoming call get routed into a conference room upon which the conference room will ring the extensions of my boss and his assistant. Not sure how I could get the assistant’s extension to stop ringing when boss answers. Also not sure how to handle a second incoming call when a first call is in progress. I would need the second call to go into a new conference room. What I would really need is a temporary conference room get created on the fly for each incoming call.

Anyone have any advice on how I could implement this?