The authors have calculated the phonon dispersion curves of barium titanate (BaTiO₃) in the cubic phase. A nonlinear shell model is used, in which the fourth-order anharmonic core-shell interaction of the oxygen ion is considered within a self-consistent phonon approximation. The temperature dependence of the ferroelectric soft mode is reproduced by their calculations, leading to the linear and nonlinear contributions of the O polarisability. The values of the relevant model parameters are critically analysed.