To my knowledge, you can't do what you want, at least not for free. SVSIP can let you make and receive VOIP calls if you pay for an SIP service, though you can just keep making new accounts to get more free trial minutes if you have the patience for it. There have been a couple of Internet chat applications for the DS, but they require both people to be running the program on their DS systems. There is no Skype client for the DS, as it's a closed protocol, and I don't think there's any way to interface with the voice chat features of any other IM clients.