NSA – Connected VoLTE – DNB to MNO – Call Flow 5G 4G DNB eNB MNO MME MNO IMS MNO LTE Service Request (RRC est. cause: MO voice Call) Initial Context Setup Request (E-RAB to be Setup List - QCI:9* & QCI:5 bearers) Initial Context Setup Response (E-RAB Setup List - QCI:9* & QCI:5 bearers) IMS Call establishment SIP: INVITE / Status: 100 Trying / Status: 183 Session Progress/ PRACK .. E-RAB Setup Request (E-RAB to be Setup List - QCI:1 bearer) E-RAB Setup Response (E-RAB Failed to Setup List - QCI:1 bearer*) RRC Connection Reconfiguration (A5 MeasConfig) * With cause value “S1 intra system handover triggered” RRC Connection Reconfiguration Complete Case 1: Valid Target cell is found Case 2: Valid Target cell is not found (A5) Measurement Report S1 Handover of QCI: 9* & QCI: 5 Bearers; QCI :1 bearer is established in MNO LTE RRC Connection Release (Redirected Carrier Info EUTRA) Release with Redirect to the Highest Priority Frequency * Assuming QCI9 is used as the default bearer CSFB Signaling