Fatto generale: 6381 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 6381 is the smallest value of n for which n and 9n together use each digit 1-9 exactly once.
Trivia: 6381 is an unremarkable number.
Data storica: June 20th is the day in 1877 that Alexander Graham Bell installs the world's first commercial telephone service in Hamilton, Ontario, Canada.