There is a rumors saying that Vonage is currently working on software that will turn PSP or Nintendo DS into a VoIP phone. According to T3 who did have a voice conversation with Mr X, they had been told that the company is in the process of creating an interface.

Both devices are compatible with VoIP, as they come with microphones and can connect to the internet via Wi-Fi. All they need the necessary interface and software. Probably it could be brought to the PSP in the form of a firmware upgrade and to the DS in a cartridge but at the moment Vonage themselves declined to comment on this matter.

[Via T3]