The ISOCS calibration method is a convenient tool for calibrating the detector efficiency as a function of energy for a wide variety of source geometries and activity distributions. The ISOCS method consists of a Canberra characterization of the detector, user input of source geometry data, and the ISOCS software which uses these to produce the efficiency calibration. During the characterization, an MCNP model of the detector is developed. The model is then independently validated using measurements with a NIST traceable source. Given the validated model, the response characteristics of the detector are mapped out to cover any location inside a sphere of radius 50 m, centered on the detector, and over a photon energy range of 50 keV-7 MeV. The ISOCS software contains a series of mathematical models that can simulate a wide variety of sample shapes. The software divides each source region into a number of voxels. Inside each voxel, a point location is defined in a quasi-random fashion. At a given energy, the detector efficiency is calculated for each voxel, taking into account the attenuation due to absorbers both inside and outside the source. The efficiencies for all the voxels are summed up at the given energy. To determine the accuracy of this calibration method, a large number of tests (about 109) were performed. In each of these tests, a reference efficiency calibration was compared to an ISOCS efficiency calibration at the same geometry. The reference calibration was either from a full MCNP calculation, or from a multi-energy radioactive source. The tests were categorized into three different counting geometries, namely, Field, Laboratory, and Collimated geometry. The data for each geometry were further divided into low energy (<150 keV) and intermediate to high-energy (>150 keV) groups. The mean ratio of ISOCS/True efficiencies was (i) 1.01±0.007 for the Field geometries, (ii) 0.97±0.007 for the Laboratory geometries, and (iii) 1.09±0.014 for the Collimated geometries. By analyzing the relative uncertainties in the True efficiencies, and the relative standard deviation in the ratios, the average relative standard deviation due to ISOCS is estimated to be 6.5%, 5.4%, and 10.5%, for the Field, Laboratory, and Collimated geometries, respectively. Various sources of bias affecting the data have been identified from this validation process. Improvements have been made in the characterization process and in the algorithms, which will be implemented in future versions of the ISOCS efficiency calibration software.