While the iPhone doesn't support Voice Over IP (VoIP) as a default feature of the phone, you can purchase and download third-party applications from the iPhone's App Store. These applications use the steady connection of your Wi-Fi connection on the iPhone in order to provide clear-quality phone calls. These applications are available for free or for a nominal fee to download, and some offer a pay-as-you-go option.
Use iCall. This free VoIP application for the iPhone provides free incoming and outgoing phone calls and allows you to talk for 3 minutes at a time before disconnecting. While this product works using your 3G network, it works best with Wi-Fi, as connection speed and quality depends on the strength of your signal. Additional features include the ability to switch phone calls from cell to the iCall app when you receive them, and unlimited calling and a personal phone number when you sign up for the Unlimited computer and iPhone plan.
Use TruPhone for iPhone. Make phone calls on your 3G network or using your Wi-Fi connection with the TruPhone application. This product allows you to place phone calls and provides three different plans depending on your frequency of use. Other features include international calls and free phone calls when calling other TruPhone, Skype or Google users. TruPhone also allows you to instant message and send Twitter updates through the application. TruPhone also offers the TruUnlimited plan, which provides unlimited phone calls to any landline in the world, and supports unlimited calls to mobile phones in nine countries.
Use Skype for iPhone. Similar to the computer version of this program, Skype for the iPhone allows you to send and receive calls from other Skype users for free, and provides non-Skype mobile calling for a small fee. Other features include texting, and provides the option to use Skype as a Pay as You Go service, or sign up for a subscription service that offers three plans based on your usage. In order to use the program you must be connected to a Wi-Fi network, as Skype is not allowed to use your iPhone's 3G network.