While Viber will allow you to call and text over wifi, it does require the receiving end to also have Viber.
 If you've never used the app before, you'll be prompted with a welcome screen - Hit "Continue."

, Click "Agree" when prompted.
 Then, type in the activation code that's texted to you.
 You can also click the Facebook button if you want to connect to your Facebook friends.
 Once your account is set up, tap the name of the person you want to call. The call should then function just like a call over a cellular network.
Make sure the person you are calling has Viber.
Open Viber.
Allow access to your address book.
Enter your phone number.
Hit continue.
Click on the contact you want to call.
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