When you call someone using mobile phone, it converts your voice into an electrical signal, which is then transmitted as radio waves and converted back into sound by receiver's mobile.
Now, in order to remain portable, mobile phones need to have relatively compact antennas and use a small amount of power. This means that mobile phones can send a signal over only a very short range.
However it is cellular network which enables you to spread these signals by dividing up land into hexagonal areas. These areas pick up the weak signal from your phone and relay it onward to another hexagon. And if you’re on the move while you talk, your phone switches hexagon as you go without interrupting your call.