Raman spectroscopy together with density functional calculations were used to study lattice dynamics in a layered honeycomb cobaltite Na3Co2SbO6, which can host a field-induced phase related with the Kitaev physics. We show that there develops an additional mode well above Neel temperature (at K) at 525 cm–1, which origin can be related to electronic excitation to one of doublets. Moreover, our theoretical calculations demonstrate that the highest frequency intensive mode related to the oxygen vibrations is very sensitive to type of the magnetic order. Thus, we propose to use the softening of this mode as a hallmark of the transition to a fully polarized regime, which is stabilized in Kitaev materials in strong magnetic fields.