Voxel phantoms and Monte Carlo methods are of considerable interest in calibrating in vivo counting applications for radioactive body burden such as whole-body counters. At the Japan Atomic Energy Research Institute (JAERI), a calculation code--UCWBC code--for whole-body counter calibrations using voxel phantoms has been developed as an EGS4 Monte Carlo user code. To validate the UCWBC code for calibrating whole-body counters at JAERI, response functions and counting efficiencies of a p-type high-purity Ge (HPGe) semiconductor detector used for the whole-body counter were evaluated for a water-filled block-shape phantom by use of UCWBC code and were measured by experiments. The voxel version of the water-filled block-shape phantom based on an actual phantom was used for the calculations. Furthermore, counting efficiencies of the Ge semiconductor detector for the male and female voxel phantoms developed at JAERI were evaluated in the photon energy range from 60 to 1836 keV by the UCWBC code in order to examine the differences between the counting efficiencies for voxel phantoms. In conclusion, it was found that the response functions and counting efficiencies of the Ge semiconductor detector found by the UCWBC code for the water-filled block-shape phantom are in good agreement with measured data. The UCWBC code was validated by those comparisons. It was also found that the counting efficiencies of the Ge semiconductor detector depend on the size of the phantoms and the effective distance between phantom and detector. The calibration of whole-body counters using voxel phantoms and Monte Carlo methods would be quite useful for the improvement in the accuracy of measurement results.