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A simulation of diurnal variation of the phase of VLF field was carried out to investigate the phenomenon of cycle slipping which had been previously observed on transequatorial paths of southwest direction. We present a simulated curve of the diurnal phase variation demonstrating the slipping phenomenon. The simulated phase curve has a part where phase changes so greatly during a few minutes that it produces a cycle slipping. We give a qualitative explanation of this phenomenon in the frame of a crude adiabatic approach. We demonstrate that at just the moment of the abrupt change of the calculated phase at the receiver the main modes degenerate at some point on the path. The realignment of the distance dependencies of phase velocities and attenuation rates of the degenerating modes at the moment of degeneration is found to be a cause of the jump of the phase. We present also a quantitative description of the phase anomalies involving a consideration of the mode conversion.