this is not a phone feature but a carrier specific feature. sprints network has to be set up to allow one to be on a call and ,say, have call waiting come in, then ADD the call waiting to the original call for conference. thats a carrier network feature.. has nothing to do with the phone having a certain ROM or update.
i dont think sprints network does this. the only thing you can do on sprint is make a conference call by calling a person, put on hold, then add another person. no merging calls or dropping a specific call. the folks you call must hang up on their own...