An
exclamatory used when you answer the phone and no one is on the other line. Usually used after a few doubtful "Hello's" have confirmed that there is in fact no one there. Reciting the word not only relieves some tension created by the situation, but also acts as a code to let the other person know you couldn't hear them, in the event that they are actually
on the line and there is just a problem with
the connection. Thus making the explanation of the abrupt hang up simpler.