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SIP ↠  ExamplesGrammarsPresence ⇾  MainServicesTransactionDialogIMS
TS 24.228§7.2.3§7.3.5§7.4.5   List   Prev   None
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IMS S5 signalling flow - Session Initiation: Mobile origination and termination in home network, but terminating UE roaming and busy - IMS S5-2c. SIP 486 Busy Here response: from S-CSCF2 to I-CSCF2, followed by ACK