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SIP ↠  ExamplesGrammarsPresence ⇾  MainServicesTransactionDialogIMS
TS 24.228§6.2   List   None   Next
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IMS R1 signalling flow - Registration: User not registered - sequence chart for IMS R1-1. SIP REGISTER request

UE1 is located in a visited network, and determines the P-CSCF via the CSCF discovery procedure.