wake-up call

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A telephone call that one requests to occur at a specific time in order to be woken up, especially in a hotel.
• noun A sign or warning that alerts one to negative or dangerous behavior or circumstances.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A telephone call to awaken someone at a certain time, especially one requested by the person while staying at a hotel.
• noun figuratively An alert, reminder, or call to action caused by a dramatic event.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a warning to take action concerning something that was overlooked or neglected
• noun a telephone call that you request be made a specific time in order to wake you up at that time (especially in hotels)