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IMS R7 Registration signalling flow - Network-initiated deregistration upon UE roaming and registration to a new network - IMS R7-1d. SIP REGISTER request (via P-CSCF2): between I-CSCF1 and HSS
  • I-CSCF1 sends the Cx-Query signalling flow to the HSS (Visited Network Identifier, subscriber identity, home domain name,). Because user has not deregistered with its previous network, HSS finds a S-CSCF assigned for that user and treats this as a re-registration procedure. Therefore, the HSS returns S-CSCF1 name to I-CSCF1.