The present invention relates to assigning an S-CSCF to an un-registered user in an IMS network. A HSS maintains information about a registration state of users and about assignments of S-CSCF to the users. HSS determines if S-CSCFs are available to serve the users. When receiving a request associated with a user and requesting address of an S-CSCF assigned to the user, assignment of the S-CSCF for the user is removed from HSS if the S-CSCF has been determined not to be available and if the registration state of the user indicates that the user is not registered. The HSS may transmit Server Capabilities to an I-CSCF to enable the I-CSCF to re-select new S-CSCF to serve the user.