The beep tone in telephone communication is an audible signal to the calling party in the form of a long high-pitched beep sounds separated by silence. This means that the connection request has failed and the call is not going through to the other person. The beep tone has become an uncommon occurrence since the invention of call waiting and voicemail, the features which are activated instead of a beep tone. The reasons behind a beep tone can be many such as the called party being busy on another call, or outside network coverage area or they might even be calling the caller at the same time.
For example, if you call your friend who is in an area where the network coverage is low or none, you will hear a beep tone and eventually the call will disconnect. Also, if you call somebody who is calling you at the same time, you will hear the beep tone.
In these modern times of mobile phones, beep tones are very rare as they have been replaced by pre-recorded messages such as “the person you are calling is out of network area” or “the person you are calling busy” based on the scenario.