Having a signal which is digital and periodic, like a phone ringing, how can you easily detect whether it is still there? You can detect the first change ie 0 -> 1 but say you want to do some stuff and abort it as soon as the signal isn't there anymore how do you find out?

Detection of zero at a time t might give you the time when the signal is being switched off but also when there is just zero instead of the signal ending.