You can place calls over the Internet when connected to a Wi-Fi network, rather than over your mobile network. To do so, you must first configure your phone to use an Internet calling account.
After you set up the account, placing an Internet call is no different than placing any other call. However, you must be connected to a Wi-Fi network. When you're making an Internet call, the phone displays Internet Call near the top of the screen.
Internet calling is based on the Session Initiation Protocol (SIP) for voice calls on Internet Protocol (IP) networks.
To add an Internet calling account and configure your incoming and outgoing preferences:
You'll also want to add SIP addresses for the contacts you want to call this way. To do so: