  Raman spectroscopy together with density functional calculations were used to
study lattice dynamics in a layered honeycomb cobaltite Na$_3$Co$_2$SbO$_6$,
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 $\approx
200$K) at 525 cm$^{-1}$, which origin can be related to an electronic
excitation to one of $j_{3/2}$ 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.
