ringback tones
A Ring-back Tone (RBT), or Audible Ringing Tone or Ring-back Signal , is the audible ringing that is heard on the telephone line by the calling party after dialing and prior to the call being answered at the receiving end. This tone assures the calling party that a ringing signal is being sent on the called party's line, although the ring-back tone may be out of sync with the ringing signal.
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Tone is the use of pitch in language to distinguish words. All languages use intonation to express emphasis, contrast, emotion, or other such elements, but not every language uses tone to distinguish lexical meaning. When this occurs, tones are phonemes (discrete speech sounds), just like consonants and vowels, and they are occasionally referred to as tonemes .
A slight majority of the languages in the world are tonal. However, most Indo-European languages, which include the majority of the most widely-spoken languages in the world today, are not tonal, with the exception of the Indo-Aryan language Punjabi.
The way in which tone is used in a particular language leads to the language being classified either as a tonal language or a pitch accent language. In a prototypical tonal language such as Chinese, the tone of each syllable can be independent of the other syllables in the word, and many words are differentiated only by the tones associated with them. In many African tone languages, since words are longer, there are fewer minimal pairs for tone, and tone may not be assigned to every syllable of a word. In a pitch accent language, there is typically only one tone-accented syllable or mora per word. For example Somali has one high tone per word. In Japanese, pitch accent refers to a drop in pitch; words contrast depending on which syllable this drop follows. Some words in Japanese contain no pitch accent at all. While many linguists maintain a difference between tone languages and pitch accent languages, linguist Larry Hyman has argued that there is no prototypical pitch accent language and that all languages that use tone phonemically should be classified as tone languages.