A method includes receiving, by a mobility management entity (MME), a redirection request message from an access and mobility management function (AMF) node, where the redirection request message includes key-related information. The method also includes generating, by the MME, an encryption key and an integrity protection key based on the key-related information. The redirection request message is used to request to hand over a voice service from a packet switched (PS) domain to a circuit switched (CS) domain.