Photovoltaic oscillations due to interband magneto-optical transitions have been observed on the quaternary open-gap semimagnetic alloys Hg_1-x-yCd_xMn_yTe. In addition to the conventional transitions, phonon-assisted interband excitations have been observed for the first time on a semiconductor. It is argued theoretically that the resonant phonon-assisted interband transitions must originate from the camel-back shaped Landau subbands. Consequently, the authors observations confirm the existence of such subbands in the Gamma ₈ valence bands of semimagnetic materials. The determined optic phonon energies correspond to the longitudinal HgTe-related mode.