Sorry, should have said. These will be the last two letters of the a/c registration. JAL towers will use "JAL xxx" where xxx is the last three numbers of the reg, and United and American will use "American xxx" Where xxx is the three numbers in the reg. (i.e. N778AN will be American 778)
Don't suppose there's ever been a situation where confusion was caused because the registration number was similar to a flight number?
JAS