Method for keeping in circuit switched fallback CSFB network and user terminal

1. A method, comprising:

obtaining, by a user terminal, a first call request message;

sending, by the user terminal, a circuit switched fallback (CSFB) request to a 4G network according to the first call request;

receiving, by the user terminal and from the 4G network, a target frequency channel number of the target network according to the CSFB request; and

accessing, by the user terminal, the target network according to the target frequency channel number

after accessing the target network, obtaining, by the user terminal, a location area of the target frequency channel number according to the target frequency channel number;

determining, by the user terminal, whether the location area of the target frequency channel number is the same as a location area of the 4G network;

detecting, by the user terminal, a call setup message if the location area of the target frequency channel number is the same as the location area of the 4G network;

performing, by the user terminal, a location area update (LAU) operation on the target frequency channel number if the location area of the target frequency channel number is not the same as the location area of the 4G network;

determining, by the user terminal, whether the call setup message is received; and

in response to determining that no call setup message is received, camping, by the user terminal, on the target network.

2. The method according to claim 1 , wherein after the camping, by the user terminal, on the target network, the method further comprises:

determining, by the user terminal, whether a preset fast return (FR) condition is met; and

performing, by the user terminal, a call operation in the target network if the user terminal receives a second call request message before meeting the preset FR condition; and

performing, by the user terminal, FR to access a 4G network if the user terminal does not receive a second call request message before meeting the preset FR condition.

3. The method according to claim 2 , wherein the preset FR condition is a preset time period; and

wherein the determining, by the user terminal, whether a preset FR condition is met comprises:

detecting, by the user terminal, an idle access time since the user terminal accesses the target network, wherein the idle access time is a time period during which the user terminal does not make a call or receive a call; and

determining, by the user terminal, whether the idle access time reaches the preset time period, wherein the preset FR condition is met if the idle access time reaches the preset time period, and wherein the preset FR condition is not met if the idle access time does not reach the preset time period.

4. The method according to claim 1 , wherein after the camping, by the user terminal, on the target network, the method further comprises:

obtaining, by the user terminal, an inter-RAT reselection instruction generated by a user operation, wherein the inter-RAT reselection instruction is an FR instruction or a hold instruction, wherein the FR instruction is used to instruct the user terminal to access a 4G network, and wherein the hold instruction is used to instruct the user terminal to camp on the target network;

determining, by the user terminal, whether the inter-RAT reselection instruction is the FR instruction; and

if the inter-RAT reselection instruction is the FR instruction, performing, by the user terminal, FR to access the 4G network; and

if the inter-RAT reselection instruction is not the FR instruction, camping, by the user terminal, on the target network.

5. A user terminal, comprising:

a radio frequency (RF) circuit, the RF circuit configured to obtain a first call request message; and

at least one processor, the at least one processor configured to send a CSFB request to a 4G network according to the first call request by using the RF circuit, wherein the 4G network delivers a target frequency channel number of the target network to the user terminal according to the CSFB request;

the RF circuit is configured to receive the target frequency channel number;

the at least one processor is further configured to:

access the target network according to the target frequency channel number;

obtain a location area of the target frequency channel number according to the target frequency channel number;

obtain a location area of the 4G network;

determine whether the location area of the target frequency channel number is the same as the location area of the 4G network;

detect a call setup message when the location area of the target frequency channel number is the same as the location area of the 4G network; and

perform a location area update (LAU) operation on the target frequency channel number when the location area of the target frequency channel number is not the same as the location area of the 4G network;

determine whether the call setup message is received; and

when the call setup message is not received, camp on the target network.

6. The user terminal according to claim 5 , wherein

the at least one processor is further configured to:

determine whether a preset fast return (FR) condition is met;

perform a call operation in the target network when receiving a second call request message before the preset FR condition is met; and

perform FR to access a 4G network when not receiving the second call request message before the preset FR condition is met.

7. The user terminal according to claim 6 , wherein the preset FR condition is a preset time period; and

the at least one processor is further configured to:

detect an idle access time since the user terminal accesses the target network, wherein the idle access time is a time period during which the user terminal does not make a call or receive a call;

determine whether the idle access time reaches the preset time period; and

determine that the preset FR condition is met when the idle access time reaches the preset time period; or

determine that the preset FR condition is not met when the idle access time does not reach the preset time period.

8. The user terminal according to claim 5 , wherein

the at least one processor is further configured to:

obtain an inter-RAT reselection instruction generated by a user operation, wherein the inter-RAT reselection instruction is an FR instruction or a hold instruction, wherein the FR instruction is used to instruct the user terminal to access a 4G network, and wherein the hold instruction is used to instruct the user terminal to camp on the target network;

determine whether the inter-RAT reselection instruction is the FR instruction;

perform FR to access the 4G network when the inter-RAT reselection instruction is the FR instruction; and

camp on the target network when the inter-RAT reselection instruction is not the FR instruction.