Could a radio station use a burst of what would sound like static or high-pitched squeals to the human ear or maybe even make it in audible but it could be picked up by your phone's microphone and then show you notification list of ads for the time that you been listening to the radio station. Better yet put it in the ultrasonic range so humans don't normally hear them.
Then if he tapped on the ad notification it would take you to the website or take you to that specific web-based ad so that you could either participate or not and if you didn't want to see the ad you got it in a notification you just swipe it away.
Could be something worth looking at!
