Okay, this may be a stupid question but I figure it's one worth asking. I've also heard carriers refer to this feature as "Sprint to Sprint" or "Verizon to Verizon" but the whole "Mobile to Mobile" thing throws me off. Is it only free to call/receive calls from people on the AT&T wireless network, or is it free to call/receive calls from ANYONE on ANY wireless network (sprint, verizon, nextel, etc.)?