Using a numerical three-dimensional MHD modelling of magneto-acoustic wave propagation in a realistic magnetostatic sunspot model we investigate the influence of the magnetic field on the parameters measured by local helioseismology. We find that the variations of temperature as well as the presence of the magnetic field cause important changes to the wave travel times. Magnetic field speeds up the waves to considerable amount, while the temperature depression within a sunspot causes the opposite action. The calculated travel time differences between the unmagnetized and magnetized atmospheres lie in the range typically obtained from local helioseismology correlation analysis. Our numerical results are also in agreement with the analytical calculations of the travel times applying WKB technique.