Phase modulation characteristics of dual-frequency liquid crystal materials possessing the dielectric anisotropy sign inversion have been studied in detail under the quasi-static and dynamic electric field control. It was found that the main obstacle in development of dual-frequency liquid crystal modulators having simultaneously short response time and large switchable values of phase retardation is related to an electrohydrodynamic instability, which disturbs the director reorientation. We are discussing general conditions at which a stable 2 pi phase modulation is possible at highest switching frequencies. In particular, for the developed dual-frequency liquid crystal material we have established conditions at which the stable phase retardation dynamic range of 2 pi at switching time similar to 2ms is realized at room temperature.