UK telecommunications operator BT has developed software for PSP that will enable Sony's handheld to call other participating PSPs and some BT phones in the UK, and eventually PCs, landlines and other cellies in as many as 100 additional countries. Using the Go!Cam peripheral (currently scheduled for a UK release on May 25), the service will utilize BT's 21 Century Network, which has been designed for IP technology -- supporting both voice and video calling. BT plans to reveal further details during the Leipzig Games Convention in August, but the service's initial launch will be limited to UK calls to and from home or BT-operated wireless hotspots (there are roughly 2,000 scattered in and around airports, train stations, hotels and fast food joints in the UK).
BBC recalls that Nintendo was the first to has also attempted to transform a portable gaming device into a clunky, crippled phone, revealing DSpeak in 2005, which enabled Mario-mimicking VoIP calls -- but the application was never released. Still, with a headset, DS users can make calls to friends while both are playing VoIP-compatible games like Metroid and Pokémon.
Update: PSP games like SOCOM Fireteam Bravo also support player-to-player VoIP communication via a headset.