Just an observation, Flight 881 leaves Boston at 9:00 am. The previous day 881 is still in the air (after accounting for time changes) is still in the air NRT-BKK. Even though it is 8000 miles away would they still use a differnet call sign. If you are talking about flight number utlization, this flight and UA 882 would be well up there.
Flight 881 is scheduled to be in the air for 22 hours and 5 min of every 24 hours. With breaks it is scheduled to be in use for 25 hours and 30 min of every 24 hours. Is their any other flight number that comes close to this ratio?