In this paper we show how inter-technology interference can be exploited to set-up a low-rate bi-directional communication channel between heterogeneous technologies, which coexist in ISM bands. In particular, we focus on WiFi and ZigBee networks, whose high density deployments make coexistence a critical issue. We monitor the transmission duration of the interference and, after recognizing ZigBee interference from WiFi off-the-shelf receivers, we precisely measure the channel busy intervals to map time duration to communication symbols. A similar approach is used on the ZigBee receivers for making the communication channel bidirectional. Extensive experimental results show the feasibility of the inter-technology communication channel. As a possible application, we designed and implemented a cross-technology TDMA scheme, alternating channel intervals to WiFi and ZigBee nodes. This unconventional communication channel can be very useful not only for coordinating channel access between WiFi and ZigBee networks, but also for other direct link applications, such as reading measurements from ZigBee sensors, or configuring ZigBee actuators (e.g. an on/off power switch) by just using common smartphones or laptops which are only equipped with WiFi interfaces.