I usually have a wake up call as a back up ten minutes after the clock radio's or my own wind-up... But some of these new-fangled clock radios that the hotels have started using are impossible to figure out.

About 35 years ago I was in Charleston WV and asked for a 6:00am wake up call. Wound up waking up myself at about 10:00am (late night working) When I called the front desk and asked what happened to my wake up call, they said it was still buzzing.
Turns out there was a quarter-sized speaker built into the wall that you could just hear if you put your ear next to it. The air conditioner probably made more noise.