A few weeks ago Gizmo Project made its debut on the Nokia 770. There are rumors of Zune being voice enabled. There is now word that Nintendo DS will allow you to make voice connections, perhaps with folks who are playing with each other over ad-hoc wireless connections.
Given that DS has WiFi, and an available
embedded Opera browser, the easiest way would be to develop a web-based SIP client that would allow you to initiate phone calls from within the browser.
Alternatively Nintendo could (and should open) up its DS platform to outside developers who could come-up with a Voice app that allowed you to make phone calls. A Skype or a Gizmo Project port to Nintendo DS would be nice!
Update: Readers reminded us that the Opera browser is sold seperately for now.