Mathematical calibration is an increasingly popular technique among laboratories with a whole- or partial-body counters. A mathematical calibration employing a voxel phantom and Monte Carlo radiation transport code simulations has many benefits and can overcome many limitations of detector efficiency calibration using physical anthropomorphic phantoms. This publication tries to identify key factors for detector modelling. The influence of such parameters depends on energy and thus is studied in the gamma energy range of detectable radionuclides, i.e. from 15 keV to 1.5 MeV.