Callback simply means returning a phone call. In business it is the everyday "I'll call you back," but in the call center industry it refers to a specific service feature: when queues are long, the center offers to "call you when it is your turn" so the customer can hang up instead of waiting on hold.
Call center callbacks improve satisfaction considerably. Customers are spared the stress of waiting through hold music and can get on with other work, while the company avoids overloaded lines at peak hours and can level out operator workload. Amazon and Apple customer support have both taken up this approach as of the 2020s.
Callback is also an effective fraud defense. If a call seems suspicious, say you will call back, hang up, and dial a legitimate number you already know; that alone exposes spoofing. Even when caller ID spoofing displays a bank's number, looking that number up yourself and dialing it connects you to the real institution. One caveat: calling back a number the caller read out to you, or the number left in your call history, is no protection at all. Check the number printed on your passbook or the back of your card, or the one on the official website, and dial that.
International callback services were a cost-saving trick of the 1990s and 2000s, when cross-border rates were high. The idea was to place a call from the expensive country, hang up at once, and have the cheaper side ring back. As IP telephony and internet calling narrowed the gap between countries' rates, the practice faded from view. The habit of returning calls is still exploited, though: one-ring scams bait victims into dialing back expensive international numbers.