Video telephony is increasingly being adopted by end consumers. It is extremely challenging to deliver video calls over wireless networks. In this paper, we conduct a measurement study on three popular mobile video call applications: Face-Time, Google Plus Hangout, and Skype, over both WiFi and Cellular links. We study the following questions: 1) how they encode/decode video in realtime under tight resource constraints on mobile devices? 2) how they transmit video smoothly in the face of various wireless network impairments? 3) what is their delivered video conferencing quality under different mobile network conditions? 4) how different system architectures and design choices contribute to their delivered quality? Through detailed analysis of measurement results, we obtain valuable insights regarding the unique challenges, advantages and disadvantages of existing design solutions, and possible directions to deliver high-quality video calls in wireless networks.