 In this case, the line often first gives that caller a ringback tone as if the call were being completed, misleading callers to think they may have actually gotten through the swarm of other calls that sometimes flood these lines.Within the North American Numbering Plan, telephone companies typically use special numbers for these lines, with a reserved prefix corresponding to high-capacity multi-line numbers instead of to a physical telephone exchange.In Miami, for example, numbers are 305-550-"xxxx", while in metro Atlanta they are 404-741-"xxxx".