Right now, Sprint is allowing Wi-Fi calling on the Samsung Galaxy S4 mini and the Samsung Galaxy Mega 6.3. These are certainly not two of the carrier's big-time sellers. That means it will be some time before we get an idea on how often Wi-Fi calling will be in use by Sprint customers, affecting the carrier's revenue. If you don't have either phone, don't worry. Sprint says it will add additional Android models throughout this year.
It should be noted that Sprint's Wi-Fi calling allows you to make free domestic calls and texts, but there still remains a fee for international calls.