ost Americans now have mobile phones, and a Nielsen Mobile report last year found that nearly one in five of us have cut the cord, abandoning our landline service entirely. Danny Kessler of Tempe, Arizona, is one of those people, except he has gone the next step: He recently gave up his cell-phone contract too. Kessler's no hermit: He's a 27-year-old personal-safety instructor who has to be in touch with his clients. He just does all his telephoning via the internet. Today Kessler is an anomaly, but internet telephony (a.k.a. voice-over-internet-protocol, or VoIP) is in a position to dominate the phone business of the future just as mobile usurped the throne of the hard-wired handset.