How the call is routed from one number to another?

When a cell phone is turned on, its first task is to find out if it is within range of a cellphone tower. As indicated by the wee antenna symbol. At the antenna end, it in turn registers that you are within its call zone, and notifies this fact to a 'central call register'.

When a distant subscriber dials your number, then their call can immediately be routed in your direction. Whether you are turned on, and where you are is available from the 'central' register.