We use skype - works quite well mostly. With a Pro Account and a real phone number attached. Latency (delay) In the Mixxx forum is a discussion on this with suggested improvements (with penalties).
For a quick improvement, if both Mixxx and Airtime are on the same LOCAL network or machine..... max out the bitrate from Mixxx - e.g. 360kb. This fills one of the buffers far quicker.