A casing for an auxiliary device to optically display an incoming telephone call includes a bottom portion which is detachably connected to a cover portion. Accommodated in the so-formed interior space is a frame which carries a plurality of reflectors spaced along its periphery and reflecting the light towards the outside through a viewing window of the cover portion. For supporting a print card within the interior space, the frame is provided with inwardly projecting clamps each of which having two shanks to hold the print card therebetween.