1. The driver of the car shown here wants to contact his friend, so he dials the number without stopping the car. He does not know where in the city his friend is.
2. The telephone instrument transmits the dialed number which the transmitter-cum-receiver (the tower) of that cell catches and sends to the mobile telephone exchange through underground cables.
3. Now it becomes the task of the telephone exchange to search for the friend. Hence, it would send the number to transmitters in all the cells. For this it uses underground cables instead of radio waves. After this each transmitter has to find out whether the holder of the dialed number is anywhere in the cell. For this every transmitter transmits the number through radio waves the very next moment. Now in which ever cell the friend is, his phone is bound to ring!
4. The friend is contacted in the cell far away! Though he is journeying several kilometers away in the opposite direction, he gets the call. Conversation begins between the two friends!
5. Here comes the interesting thing. The friend’s car is riding towards the limit of the cell, i.e. the border of the cell. It is clear that the car will cross the border in a while – and after that the transmitter in the cell will not be able to catch the radio waves of the conversation; nor will it be able to send any. But if the conversation between two persons is cut off in middle of the talk then what is the use of mobile phone! Hence, as soon as the car crosses the border of its cell the transmitter hands over the task to the next transmitter. While handing over if a sentence is unfinished the remaining words are transmitted by the next transmitter! And it is done so quickly that the speaker does not even realize that the transmitter is changed.
As against such benefits of a mobile phone it is true that a phone itself is a transmitter, and when held against an ear the powerful radiofrequency energy and the magnetic field that is created through it may harm the brain – but the research on that aspect still awaits conclusion.
Mobile phone (Wikipedia)