Techniques in a mobile device for use in initiating vertical handover of a voice or a data call between a wireless local area network (WLAN) and a wide area wireless network (WWAN) are described. The mobile device performs communication operations for maintaining the voice or the data call via the WLAN or the WWAN. During the voice or the data call, the mobile device identifies a WLAN signal quality value of signals with the WLAN, as well as a WWAN signal quality value of signals with the WWAN. The mobile device then performs a comparative evaluation of the WLAN and the WWAN signal quality values, and initiates a vertical handover of the voice or the data call between the WLAN and the WWAN based on the comparative evaluation of the WLAN and WWAN signal quality values. The comparative evaluation may involve calculating a signal indicator value which is based on a ratio of the WLAN and the WWAN signal quality values. Alternatively, the comparative evaluation may involve comparing or finding the difference between the WLAN and the WWAN signal quality values.