A mobile phone's built-in address book has limited usefulness. For instance, it only allows users to call people whose numbers they've already entered – which means people they know well. Directory assistance, on the other hand, is about reaching people or establishments one knows almost nothing about. But there's no easy way to deal with the vast territory in between – to reach people one knows only casually, or to learn more about people and companies one knows almost nothing about. San Francisco-based CallSpark, which presented at the recent DEMO conference, is trying to fill that gap.