The low-frequency dynamics of (Formula presented) has been investigated in the temperature range T=300-15 K. The dispersion surfaces at room temperature display a deep valley located along the (0.5, ζ, 0.5) zone-boundary line. This valley becomes deeper when T decreases, showing a soft-mode behavior. In addition a new depression appears at low-temperature along the (ξ, 0.5, 0.5) line, corresponding to an incipient instability toward an incommensurate phase. A symmetry analysis of the vibration spectrum including the compatibility relations between the different irreducible representations, is presented. The soft-mode symmetry is determined as belonging to the (Formula presented) representation. The present results are compared to previous Raman and x-ray investigations performed on this compound.