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.