Sony PSP users can now make VOIP calls from their console via Skype for PSP, Sony in a press release announced Skype features to be made available in new PSP-2000 series(PSP Slim, PSP Lite, and PSP Petite), PSP users will be able to call free to other Skype users and calling credits can be purchased for calling on landlines and mobiles world-over, Users will need memory stick duo, Wi-Fi internet connection and a Skype-compatible microphone to make calls via PSP.

PSP users worldwide will be able to start using Skype features and
services through a simple system software update scheduled in late January.
After updating the PSP system software, a Skype icon will be added to the
Network category in the home menu. PSP users who do not yet have a Skype
username can register by clicking the icon and following the instructions.
PSP users who already have a Skype username can immediately sign in by
clicking the Skype icon and entering their username and password.

Users will be able to take benefit of following features on Skype for PSP:

• Unlimited free voice calls from one PSP to another, or from Skype to Skype.
• Skype contact management functions.
• Skype presence.
• SkypeOut.
• Skype account management to view and modify Skype settings and account details.
• SkypeIn - number to receive calls on Skype from
  landlines and mobiles anywhere in the world.

There also exist another application called "Furikup" allowing you to use SIP functionality on your PSP, developed by team N00bz the application let you use SIP based services on PSP meaning you call use your PSP to make international calls at local calls.