A core network device receives a fallback identifier from an access network device in a first radio access network when radio access technology (RAT) of a terminal is falling back from RAT of the first radio access network to RAT of a second radio access network. The first radio access network is a 5G network, and the second radio access network is a 4G network. The core network device determines, according to the fallback identifier, that the fallback of the RAT of the terminal from the RAT of the first radio access network to the RAT of the second radio access network is for establishing a voice service under the second radio access network. After the RAT of the terminal falls back from the RAT of the first radio access network to the RAT of the second radio access network, the core network device establishes a bearer/flow for the voice service.