Automatic ring back
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Automatic ring back is a service offered by phone companies. Normally, when you call someone and their line is busy, you have to call back every few minutes to check if their line is free yet. With automatic ring back, you just dial a code (*66 in some places). Then, when their line is free, your phone rings with a distinctive ring (so you know it is automatic ring back and not a regular call). When you pick it up, it calls the other number since it is now free.