The integration of cellular and voice over WLAN (VoWLAN) systems recently has attracted considerable interest from both academia and industry. A cellular/VoWLAN dual mode system helps mobile users to access a low cost voice over IP (VoIP) service in a WLAN area and switch to a wide-coverage cellular system without WLANs. However, the system suffers the power consumption problem and the service continuity issue between cellular and WLAN systems. This study presents a network architecture for an enterprise or a dual mode service provider to permit service continuity between cellular and VoWLAN systems. This architecture is used as a basis for further elaborating a novel power saving mechanism, called PIANO (paging via another radio). By applying the proposed mechanisms to a dual mode mobile, the mobile can completely switch off its WLAN interface, only leaving the cellular interface awake to listen to paging signals. When a dual mode mobile receives a paging message from its cellular interface, it wakes up its WLAN interface and responds to the call via WLAN networks. Therefore, a dual mode mobile can reduce the idle mode power consumption, and can also obtain VoWLAN services. The simulation results illustrate that the proposed methods significantly extend the standby hours of a dual mode mobile.