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According to the official Gmail Blog, this new multi-call capability will let you put callers on hold with Gmail's phone service, picking up another call at the same time. Too bad there's no music on hold to entertain that first caller.
Also imitating conventional phone systems is call waiting, where you receive a notification of an incoming call, and then it's up to you whether you're going to answer it or not. The only limitation with this new capability is that you're only allowed two outgoing calls to physical phones at the same time.