Disclosed is a method for determining Idle mode Signaling Reduction (ISR) activation in a mobile communications system.When a mobility management node decides to activate an ISR feature in an IMS network environment where hetero mobile communications networks (e.g., E-UTRAN and UTRAN/GERAN) interwork with each other, considered are not only whether an ISR feature can be supported, and whether a UE can use IMS voice, but also conditions (information) on whether IMS voice over PS domain can be supported. This may allow a voice call transferred to the UE from the network to be delivered to a domain (PS or CS domain) selected with consideration of an access network state without delay.