How to use free Wi-Fi as carrier instead of sprint on an iPhone?

Going to Bahamas and sprint charges to make calls and text. However, if I turn off sprint services, I can use Wi-Fi for free.
Can someone help me to do this? On an iPhone…

Added (1). So if I get an app, it automatically changes my phone over from sprint services to the free Wi-Fi? I did get an app, and they also charge, a very small amount, but there's a charge.

You'll need an app that lets you make calls via Wi-Fi. Skype is one - but unless you are calling someone else who is also using Skype on their phone or computer, you will need to pay Skype for the minutes (instead of Sprint). But that will still be a lot cheaper than paying Sprint for International roaming.