Although making phone calls is very common and something that we do almost on a daily basis, the steps involved in realizing calls is complex and involves several processes that work behind the scenes.

A call cannot be completed only through the use of the caller’s telephone and a central office. There are several types of signalling that is also involved during the progression of a call.

The call starts when the caller picks up the receiver of his telephone.

The following are the steps that take place from then on:

Step 1 - A signal is sent to the Central Office (CO) of the local exchange company (LEC), or local telephone service company, serving the caller, that is, the company to which you pay your phone bills. This is called the off-hook signal.

Step 2 - A switch at the telephone company’s Central Office recognizes the signal (off-hook signal) and responds by transmitting a dial tone back to the caller. This sis the sound we hear each time we pick up the phone to make a call. Once we hear this tone, we know that we can now dial the number to be called.

Step 3 - While the number is dialed, the switch at the central office receives the digits and sends signals to different nodes within the switch that processes the requested number. This was traditionally done by switch-board operators but has been replaced by automatic switching systems. These nodes within the switch have three main functions: they provide the physical access, establish signaling and connectivity, and process the service itself.

Various switch nodes communicate with each other through this process called messaging. In some cases the central office has to hand the call off to another telephone company’s central office, due to the fact that they may not be serving the area that is being called. This interoffice communication is called signaling. The signaling that occurs between two offices includes different types of communication, such as on and off-hook, applied ringing, and central office destination or central office originator responses.

Step 4 - Once the signal arrives at the central office of the telephone company that serves the person that is being called, a ring generator is applied to the called party’s line so that his or her phone rings notifying that someone is calling. At the same time, the originating central office sends audible ring back to the person calling so that he or she is aware that the phone is ringing on the other end.

Step 5 - Once the called party lifts the telephone handset, a dedicated circuit is established between the caller and the called party for the duration of the call. Note that these call processes not only apply to voice, but that the information being sent may also be data, such as a fax signal. Communication between switches is always in digital format, so all analog data, like voice, is converted to a digital signal at one end and then back to analog at the other end.

When the call is completed, the local office’s switch at the caller’s end receives the signal that the phone set has been hung up, which is called a on-hook signal. The called person’s local office switch will also detect the on-hook signal. Once the disconnection is detected, call records are updated and the information is stored for billing of the call to the phone number of the person that called.