If you just want to make free unlimited calls anytime to anyone in the US from your mobile phone. Just use the voicemail trick on sprint
or you can use the grandcentral trick.
http://forum.ppcgeeks.com/showthread.php?t=24644 (ps not trying to promote my thread or anything just happend that the topic has to do with this thread topic).
Also if you wanted to call international with your mobile. You can also just use Gizmo project which is a free Voip service set it up on your mobile with a softphone. The provide free incoming calls but have prepaid rates for calling out. Their rates are pretty close if not cheaper for certain countries than the device. Since Voip uses data if you have unlimited data sprint won't charge you for the call.