NYT Article on Femtocells
Even though the calls would be offloaded to an Internet service provider, AT&T customers would be charged for the minutes of phone service in their existing wireless plans unless they pay an extra $20 a month for unlimited calling.
Ridiculous.
There also are ways to connect mobile phones to Wi-Fi hot spots, but that requires people to download software, like Skype, to route the calls to the cellular network.
So I can summarise what I think: I would love Apple to provide such 'ways' integrated right into the iOS Phone and Messages apps and I think they have already done most of the work with FaceTime and push notifications.