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.
