The DS is a very versatile piece of machinery. Its many capabilities are only overshadowed by the ever-increasing library of great games for it. Now, the thing can be used as a phone?
Yes, utilizing some of samuelv's hard work, you too can conduct phone conversations with just your DS and a wi-fi connection. Oh, and according to some, a knowledge of homebrew on the DS is also needed, as some have complained the software can be a bit of a pain.
Oh, and you'll also need a SIP account.