We performed factor-group analysis of all phonons in possible monoclinic C2/c and C2 structures of BiMnO3 and compared it with our experimental infrared and Raman spectra. We conclude that the crystal structure is centrosymmetric C2/c in the whole investigated temperature range from 10 to 550 K, therefore BiMnO3 cannot be ferroelectric. We revealed a dielectric relaxation in THz spectra above the structural phase transition taking place at TC1 = 475 K giving evidence in strong lattice anharmonicity and a large dynamical disorder of Bi cations above TC1. Step-like dielectric anomaly observed at TC1 in THz permittivity reminds antiferroelectric phase transition. Nevertheless, the low-temperature dielectric studies did not reveal any antiferroelectric or ferroelectric hysteresis loop. Our experimental results support theoretical paper of Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that BiMnO3 is not multiferroic but only antipolar ferromagnet.