Panasonic KX-TDA6178 24-Port SLT Extension Card with Caller ID ECSLC2424-Port SLT Card with CID (ECLSC24)Supports Caller ID compatible single line telephones (SLTs)Includes 4 power failure circuitsRequires 1 free slotOnly installable on shelves that are equipped with a KX-TDA0103 L-type power supply..