Hisex hens, aged 24 weeks, were divided into 6 groups. Each group consisted of 4 cages with 10 hens per cage with dimensions and equipment meeting the EU directives. This 2 × 3 factorial experiment included two levels of calcium (Ca; 35 or 42 g/kg) and 3 different additions of 6-phytase OptiPhos® (Ph; 0, 300, and 1500 phytase units (FTU)/kg) to the diet. The content of nonphytate phosphorus (NPP) in all diets was the same (1.8 g/kg). The experiment lasted 12 weeks. A significant interaction of Ph × Ca (P = 0.029) was observed in hen-day egg production. A diet with 35 g/kg Ca and the highest dose of Ph (1500 FTU/kg) resulted in a lower hen-day egg production (84.1%) than did the other treatments (91.5–88.6%). Phytase superdosing negatively influenced egg mass production (P = 0.001) and the feed conversion ratio (P = 0.018). Neither Ph nor Ca influenced eggshell ash content. Both additions of Ph into mixed feed decreased Haugh units (P < 0.001). A higher content of Ca in the diet increased shell thickness (P = 0.024) and shell breaking strength (P = 0.039), while Ph addition increased shell percentage (P = 0.004) and shell breaking strength (P = 0.009). The results of this experiment demonstrate the unsuitability of Ph superdosing in mixed feed for laying hens.