Apparatus comprising: one or more processors, and memory storing instructions that, when executed by the one or more processors, cause the apparatus to: instruct a user data repository to store IMS SDM subscription data, wherein the IMS SDM subscription data comprise information on a notification from the user data repository to which a call session control function subscribed by a subscription and a subscription identifier of the subscription and at least one of: • an instance identification of the call session control function; • a callback reference for the notification; • an identification of a resource of the user data repository for which a change triggers the notification; and • an expiry time of the subscription; get the IMS SDM subscription data from the user data repository for the call session control function if the call session control function has an indication that the call session control function might have lost the IMS SDM subscription data; restore the IMS SDM subscription data at the call session control function according to the IMS SDM subscription data got from the user data repository. one or more processors, and memory storing instructions that, when executed by the one or more processors, cause the apparatus to: instruct a user data repository to store IMS SDM subscription data, wherein the IMS SDM subscription data comprise information on a notification from the user data repository to which a call session control function subscribed by a subscription and a subscription identifier of the subscription and at least one of: • an instance identification of the call session control function; • a callback reference for the notification; • an identification of a resource of the user data repository for which a change triggers the notification; and • an expiry time of the subscription; get the IMS SDM subscription data from the user data repository for the call session control function if the call session control function has an indication that the call session control function might have lost the IMS SDM subscription data; restore the IMS SDM subscription data at the call session control function according to the IMS SDM subscription data got from the user data repository. instruct a user data repository to store IMS SDM subscription data, wherein the IMS SDM subscription data comprise information on a notification from the user data repository to which a call session control function subscribed by a subscription and a subscription identifier of the subscription and at least one of: • an instance identification of the call session control function; • a callback reference for the notification; • an identification of a resource of the user data repository for which a change triggers the notification; and • an expiry time of the subscription; • an instance identification of the call session control function; • a callback reference for the notification; • an identification of a resource of the user data repository for which a change triggers the notification; and • an expiry time of the subscription; get the IMS SDM subscription data from the user data repository for the call session control function if the call session control function has an indication that the call session control function might have lost the IMS SDM subscription data; restore the IMS SDM subscription data at the call session control function according to the IMS SDM subscription data got from the user data repository.