wake-up call
noun [ C ]
ukusif something is a wake-up call, it makes you realize that you need to change the way you do something, to avoid a problem in the future:
The 30% drop in annual profits served as a real wake-up call for high-street retailers.
a phone call that a hotel guest can arrange to get to wake them up at a particular time:
I'd like a wake-up call at 7, please.