Call selector is a device used in the situation that there are two outgoing lines but only one telephone set. It can reroute some calls to go through one line, and the other calls to go through the other line automatically. People have a PSTN line in use, and they might want to install another VoIP line or GSM/ CDMA FWT line as usually the rate of other line is much lower. It can help people choose the most economic call router and save call cost.