Apple’s developer documentation doesn’t go into much depth about passing voice calls through your iPhone to your Mac or iPad, but the preview site indicates that both devices will need to be on the same Wi-Fi network (having a Mac hooked to wired Ethernet will also probably work, but it's safe to assume that more homes are wireless these days). Unlike Handoff, the feature doesn’t appear to use Bluetooth at all, and unlike AirDrop, it doesn’t require your device to support peer-to-peer Wi-Fi connections. Unlike so many of the other Continuity features, this one looks like it should work fine even if you’re using an iPhone 4S with an iMac or MacBook Pro from 2007.
Given what we know about how it works and about other features being added to iOS 8, the ability to take and make phone calls from a Mac or iPad is likely an extension of the voice over IP (VoIP) capabilities that power FaceTime Audio in iOS 7 and OS X 10.9.2. Rather than sending voice over the Internet between two Apple devices, it appears to be communicating between two devices on your local network to deliver voice calls. Look at the official screenshots Apple has released to promote the feature, and you'll notice that Apple even uses the same kind of pop-up notification UI in Yosemite for both iPhone and FaceTime Audio calls.
I cannot wait for a deep dive on this technology once iOS 8 and Yosemite are released.