Special information tones
（重定向自Special Information tone）
In telephony, a special information tone (SIT) is an in-band international standard signal consisting of three rising tones indicating a call has failed. It usually precedes a recorded announcement describing the problem.
Because the SIT is well known in many countries, callers can understand their call failed even though they do not understand the language of the recorded announcement (e.g., when calling internationally) instead of assuming the recording is voicemail or some other intended function.