1. A method for use in delivering data from the host service to a wireless mobile communication device operative in a wireless communication network, the host service being configured to maintain in a host memory an availability status indicative of the availability of the mobile device in the wireless communication network, the method comprising: generating the data at the host service; identifying whether the availability status of the mobile device in the host memory is known; if the availability status is known at the host service, then: transmitting the data from the host service to a wireless router system if the known availability status at the host service equals available, so that the wireless router system is able to send the data to the wireless communication network for transmission to the mobile device; queuing the data at the host service without transmission to the wireless router system if the known availability status equals unavailable; wherein the cached availability status in the cache memory of the wireless router system is set based on an indication signal from the mobile device, the indication signal indicating one or more functional states or parameter levels of the mobile device; when the cached availability status in the cache memory equals to available: in response to the wireless router system failing to receive the indication signal from the mobile device within a predetermined interval, or in response to the wireless router system identifying that one or more functional states or parameter levels of the indication signal are indicative of unavailability: changing the cached availability status from available to unavailable and transmitting the changed availability status from the wireless router system to the host service; when the cached availability status in the cache memory equals to unavailable; in response to the wireless router system receiving the indication signal from the mobile device where the one or more functional states or parameter levels of the indication signal are indicative of availability: changing the cached availability status from unavailable to available and transmitting the changed availability status from the wireless router system to the host service; and setting, as the known availability status at the host service, the cached availability status received from the wireless router system.