A method for operating an internet protocol, IP, multimedia subsystem, IMS, of a communication network , wherein - a first terminal device connected to a communication network registers a first proxy-call session control function, P-CSCF, node of an IMS of the communication network with a serving-call session control function, S-CSCF, node of the IMS , wherein registering the first P-CSCF node comprises the first P-CSCF node to forward a registration message received from the first terminal device to a shared data layer, SDL, of the communication network , the registration message indicating a subscription to a switchover service of the IMS , and the SDL to receive and to store the received registration message and to send an acknowledgment to the first P-CSCF node ; - a second terminal device connected to the communication network sends an IMS call request to the S-CSCF node of the IMS , the IMS call request requesting an IMS call connection to the first terminal device ; - the S-CSCF node receives the sent IMS call request and forwards the received IMS call request to the first P-CSCF node ; - the S-CSCF node , upon a failure of the first P-CSCF node , forwards the IMS call request to a second P-CSCF node of the IMS ; - the second P-CSCF node receives the forwarded IMS call request , checks the subscription of the first terminal device , wherein checking the subscription comprises requesting and receiving the registration message from the SDL , and, when the first terminal device is subscribed to the switchover service of the IMS , locates the first terminal device and sends a registration request to the located first terminal device ; - the first terminal device receives the sent registration request and then registers with the second P-CSCF node ; - the second P-CSCF node forwards the received IMS call request to the first terminal device , - the first terminal device receives the forwarded IMS call request and answers the requested IMS call request . - a first terminal device connected to a communication network registers a first proxy-call session control function, P-CSCF, node of an IMS of the communication network with a serving-call session control function, S-CSCF, node of the IMS , wherein registering the first P-CSCF node comprises the first P-CSCF node to forward a registration message received from the first terminal device to a shared data layer, SDL, of the communication network , the registration message indicating a subscription to a switchover service of the IMS , and the SDL to receive and to store the received registration message and to send an acknowledgment to the first P-CSCF node ; - a second terminal device connected to the communication network sends an IMS call request to the S-CSCF node of the IMS , the IMS call request requesting an IMS call connection to the first terminal device ; - the S-CSCF node receives the sent IMS call request and forwards the received IMS call request to the first P-CSCF node ; - the S-CSCF node , upon a failure of the first P-CSCF node , forwards the IMS call request to a second P-CSCF node of the IMS ; - the second P-CSCF node receives the forwarded IMS call request , checks the subscription of the first terminal device , wherein checking the subscription comprises requesting and receiving the registration message from the SDL , and, when the first terminal device is subscribed to the switchover service of the IMS , locates the first terminal device and sends a registration request to the located first terminal device ; - the first terminal device receives the sent registration request and then registers with the second P-CSCF node ; - the second P-CSCF node forwards the received IMS call request to the first terminal device , - the first terminal device receives the forwarded IMS call request and answers the requested IMS call request . <EOS>