An intermittent audible indication to the calling party that a dialed telephone number is ringing. A ringback tone is a status indicator that the dialed number is available (i.e., not busy), that all connections through the appropriate network or networks between the originating and destination devices either have been made or are available to be made, and that the call can be connected if someone or something answers the call. The call can be answered by a human being if the destination telephone number is not engaged, or if the destination telephone number is engaged, has call waiting service, and a call waiting indicator tone prompted a human being to either abandon the first call or place it on hold in order to answer the second. The call can be answered by a premises-based answering machine or a local or network-based voice processor in the event that the called number does not answer, is busy, or does not answer within a programmable number of rings. The nature of the ringback tone varies by region and country. In the United States, Canada, and other countries in the North American Numbering Plan (NANP), the standard PSTN ringback tone is generated by summing a 440-Hz tone with a 480-Hz tone and applying these to the telephone line in a two-second on and four-second off cadence.The tone combination produces a warbling "ring ... ring ... ring" sound, caused by the 40-Hz beat, or interference due to the difference in frequency, between the two tones.The ringback tone may be generated by the switch serving either the called party or the calling party, but it is not generated by the called telephone instrument or PBX.The ringback tone generally starts and stops at the same rate as the ringing tone of the called telephone, but generally is out of phase, i.e., staggered in time. Personalized ringback tones recently have become popular, especially with respect to cellular telephones. Dozens of music genres and hundreds of selections are commonly available for both the ringback tone and the ringing tone.The service generally is on a subscription basis and carries an additional charge per tone selected. It is possible to select up to 100 or so active ringback tones at any given time, to play different tones for different callers, and to vary the tone by time of day, perhaps to have one tone for business hours and another for all other times. See also ring tone.