The ubiquity of the cell phone has finally prompted AT&T to pull out of the pay-phone business, the company announced on Monday. By the end of 2008, AT&T says, it will have exited the business completely. The nation's No. 1 wireless-network operator, AT&T says it expects independent operators to pick up service contracts on its pay phones, which, over the past 10 years, have shrunk in number to about 1 million nationwide from 2.6 million.