Does Yosemite's Continuity use WiFi or Bluetooth to carry Voice?
From ArsTechnica:


  Unlike Handoff, the feature [phone calls] 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.