As we get older, our ability to hear the higher notes is lessened, due to the aging process making the bones in the ear harder and less able to vibrate. This is the rate at which the signal changes its relationship to time, expressed as degrees. One complete cycle of a wave begins at a certain point, and continues till the same point is reached again. Phase shift occurs when the cycle does not complete, and a new cycle begins before the previous one has fully completed. The human ear is insensitive to phase shift, but data signals are severely affected by it.
Phase shift is caused by imperfections in cable media, such as joins and imperfect terminations.
In a practical sense, imagine you are in the bath. If you drop the soap, it forms ripples where the waves travel outwards towards the edge of the bath. When the wave reaches the edge, it hits the wall of the bath and bounces back. This is like phase shift, which is an abrupt change in the signals relationship. Digital signals cannot be sent via the PTSN without being first converted to analogue.