Callback means returning a phone call. In business, it's the everyday "I'll call you back," but in call centers it refers to a specific service: during high volume, offering "We'll call you when it's your turn" so customers can hang up and wait without holding.
Call center callbacks significantly improve satisfaction. Customers avoid the stress of waiting with hold music and can do other tasks. For businesses, it prevents peak-hour line overload and balances operator workload. Amazon and Apple customer support use this approach.
Callback is also a fraud defense. When receiving suspicious calls, saying "I'll call back" and hanging up, then dialing the known legitimate number, defeats spoofing. Even if caller ID spoofing shows a bank's number, independently looking up and dialing the bank connects to the real institution.
International callback services also exist, where calling from a high-rate country and having the cheaper side call back saves costs. However, one-ring scams exploit this by tricking victims into returning expensive international calls.