Using SvSIP, a homebrew application, you can get VoIP on the Nintendo DS. SvSIP is based on pjsip, an open source embedded SIP stack and media stack written in C and whose claim to fame is a very small footprint. Using any SIP-based service provider you can enter your SIP credentials and get your VoIP on. 
No word as to whether you can simultaneously VoIP using this app and play a game, which would allow you to trash talk your online gaming buddies.
You can download the software here.
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Tom,
Just a slight correction, there is a misspeling of 'pjsip' in your post.
Thanks!