The phonon spectrum of CsBi2 is qualitatively different from RbBi2. When SOC is neglected, the modes are not only softened as it is in the case of RbBi2, but the spectrum is rich in imaginary frequencies, suggesting structural instability. Inclusion of the SOC removes the imaginary frequencies, thus the relativistic effect of SOC is crucial for the stability of the cubic Laves structure of CsBi2.