LONDON (Reuters) - Cingular, the second-largest U.S. mobile phone company, has clinched AT&T Wireless, its smaller rival, after outbidding Vodafone Group Plc with a $15-per-share offer, a source familiar with the situation told Reuters. The bid values AT&T Wireless Services Inc., the third-largest U.S. mobile phone group which put itself up for sale on January 22, at about $40.7 billion. "That's what we've been told, but it's a fluid situation," the source said. AT&T Wireless and Cingular are planning to host a morning press conference in the U.S., the source said. Cingular and Vodafone declined to comment, while AT&T...