Automatic ring back is a service offered by phone companies.
On making a telephone call to a number that is engaged, automatic ring back is a service provided by the telephone company whereby, when the called number becomes available, the caller is rung back, usually with a distinctive "ring back" ring. When the phone is picked up, it calls the previously engaged number. The service is typically invoked by keying a number on hearing the engaged tone.
In the UK the service is also known as Ringback.[1]
In the United States the service is often called Auto Redial.
In the United States Army on post wide communications networks this is referred to as Callback or Automatic Callback.
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