A month ago, Brandy McDowell sat down with her longtime friend, Kezia Chandler, and told her she had switched phone carriers. Their relationship has not been the same since. Now, they barely speak. Ms. Chandler rushes Ms. McDowell off the phone when she calls during her lunch break. And long conversations about schoolwork and relationship woes have been reduced to sound bites. But what was set up as a purely business strategy is having an unintentional social effect. It is dividing the people who share informal bonds and bringing together those who have formal networks of phone “friends.”