A method for accessing a Completion of Call to Busy Subscriber, CCBS, service based on an identity, comprising: rejecting, by a callee, a call invite from a caller when the callee is busy; sending, by an Originating Application Server, O_AS, a subscribe message to a Terminating Application Server, T_AS to subscribe the idle state of the callee; sending, by the T_AS, a notification message indicating that the callee is idle to the caller when detecting that the callee is idle; authenticating, by the T_AS, the identity of a caller initiating a recall invite when receiving the recall invite and determining that the recall invite includes a completion call indicator, wherein the recall invite is the call invite once again from the caller to the callee when the state of callee is from busy to idle; allowing, by the T_AS, the access of the caller when the identity authentication is passed; and rejecting, by the T_AS, the access of the caller when the identity authentication is failed; wherein authenticating, by the T_AS, the identity of the caller initiating the recall invite based on a shared key, specifically by: the T_AS determining whether the recall invite of the caller forwarded by the O_AS includes shared key information, and sending a reject message to the O_AS when the shared key information is not included in the recall invite forwarded by the O_AS, wherein the reject message sent by the T_AS includes a challenge value generated based on the shared key; upon receiving the reject message from the O_AS, the caller resending a recall invite, which includes the shared key information, to the T_AS; and the T_AS authenticating the shared key information, wherein the identity of the caller becomes legal when the authentication is passed and the identity of the caller becomes illegal when the authentication is failed. rejecting, by a callee, a call invite from a caller when the callee is busy; sending, by an Originating Application Server, O_AS, a subscribe message to a Terminating Application Server, T_AS to subscribe the idle state of the callee; sending, by the T_AS, a notification message indicating that the callee is idle to the caller when detecting that the callee is idle; authenticating, by the T_AS, the identity of a caller initiating a recall invite when receiving the recall invite and determining that the recall invite includes a completion call indicator, wherein the recall invite is the call invite once again from the caller to the callee when the state of callee is from busy to idle; allowing, by the T_AS, the access of the caller when the identity authentication is passed; and rejecting, by the T_AS, the access of the caller when the identity authentication is failed; wherein authenticating, by the T_AS, the identity of the caller initiating the recall invite based on a shared key, specifically by: the T_AS determining whether the recall invite of the caller forwarded by the O_AS includes shared key information, and sending a reject message to the O_AS when the shared key information is not included in the recall invite forwarded by the O_AS, wherein the reject message sent by the T_AS includes a challenge value generated based on the shared key; upon receiving the reject message from the O_AS, the caller resending a recall invite, which includes the shared key information, to the T_AS; and the T_AS authenticating the shared key information, wherein the identity of the caller becomes legal when the authentication is passed and the identity of the caller becomes illegal when the authentication is failed. the T_AS determining whether the recall invite of the caller forwarded by the O_AS includes shared key information, and sending a reject message to the O_AS when the shared key information is not included in the recall invite forwarded by the O_AS, wherein the reject message sent by the T_AS includes a challenge value generated based on the shared key; upon receiving the reject message from the O_AS, the caller resending a recall invite, which includes the shared key information, to the T_AS; and the T_AS authenticating the shared key information, wherein the identity of the caller becomes legal when the authentication is passed and the identity of the caller becomes illegal when the authentication is failed.