Callcentric has recently released their new iPhone app which is now available through the iTunes store. The Callcentric iPhone app allows you to use your iPhone to place calls worldwide from your Callcentric account.
In essence, the iPhone app uses Callcentric's Click 2 Dial feature, which is generally initiated through your PC's web browser, while logged into your Callcentric account. Now, you can initiate a Click 2 Dial call directly from your iPhone.
Click 2 Dial is essentially initiating a "callback" to your iPhone phone number, or to any phone number you stipulate for the callback (i.e. your work, home, or friends phone.... where ever you happen to want to receive the callback to).
Callcentric provides more detailed information on how to download, install, and use the iPhone app for making calls through your Callcentric VoIP account.