To make low- or no-cost Voice calls over Wi-Fi, you will need an app on your phone that allows you to call over the Internet (VoIP software). I recommend Skype, if you do not already have this on your phone (some carriers, such as T-Mobile, include it in their smartphones.

The “Mobile Wi-Fi” feature that your phone has is a more advanced feature than just Wi-Fi access. Mobile Wi-Fi allows your phone to create a Wi-Fi hotspot, to share data access with other Wi-Fi devices (iPad, laptop, etc.), carrying this data your 3G data plan. You do not need this feature to use Wi-Fi to save money. In fact, sharing your 3G data plan with others can be very costly, depending on your plan–so be sure that you know your data plan limits before you use this feature, and avoid using it when roaming, as the costs can be devastating.

Save $$$ on International Roaming: You will not be charged Dollars/minute for Intl Roaming calls, only Cents/minute, by using VoIP. To maximize your savings, use (free) Wi-Fi instead of the 3G connection.
Save $$$ on International Long Distance: You will not be chanrged Dollars/minute for Intl calls, only Cents/minute, by using VoIP. Calling home from abroad can be very costly.

Caution: If you are outside of your country with a smartphone, consider using WiFi only, as international roaming data charges can be as high as $20/MB, allowing you to come home to receive a thousand dollar roaming charge. 40% of smartphone users have received a “Surprise” charge that was far larger than they expected.