Data-only mobile phone plans are inevitable, says AT&T (NYSE:T) CEO Randall Stephenson at a Bernstein conference (webcast). Under such plans, a subscriber would presumably rely on VoIP services such as Skype for calls. Stephenson adds AT&T plans to experiment with app-based data pricing and shared data plans - Verizon Wireless (VZ, VOD) has already promised shared data plans by mid-summer.