A method of supporting IMS signaling over EPS bearer or QoS flow upon inter-system change between EPS and 5GS is proposed. When inter-system change from 5GS to EPS occurs, a PDU session is converted to a PDN connection having multiple EPS bearers. The EPS bearer mapped from the QoS flow that is used for IMS signaling in 5GS is the EPS bearer for IMS signaling in EPS. When inter-system change from EPS to 5GS occurs, a PDN connection is converted to a PDU session having multiple QoS flows. The QoS flow that is associated with the default QoS rule is the QoS flow for IMS signaling in 5GS, regardless of which EPS bearer is used for IMS signaling in EPS.