In one aspect, the invention provides a method, and associated apparatus for re-registering a contact address of a user of an IP Multimedia Subsystem, IMS, network. A request for re-registration of the contact address is received at a Serving-Call Session Control Function, S-CSCF, node serving the user. After a determination that the S-CSCF has no stored information relating to the contact address, a request is sent to a Home Subscriber Server, HSS, of the user to register the contact address information. The HSS detects if the contact address information relating to the user has already been backed-up by the HSS, and notifies the node serving the user of registered contact address information backed-up by the HSS. Another aspect provides a method and associated apparatus for de-registering a registered contact address of a user of an IP Multimedia Subsystem, IMS, network.