Helio CFO Todd Tappin and execs Michael Zemetra, Terry Boyle and Kieran Hannon will leave the company by March 31. A source tells us former CEO and current chairman Sky Dayton won't remain long, either. The cell-phone carrier started as a joint venture in 2006 between EarthLink, the Internet service provider founded by Dayton, and South Korean phone company SK Telecom. Since then, it has disappointed, and EarthLink ran short on cash to invest. When SK Telecom reupped with another $270 million last fall, reducing EarthLink's share to 22 percent, this kind of shakeout was expected. In fact, our source tells us most if not all executives from the "EarthLink side of the house" will depart the company on or before March 31.