My experience with ATC is limited to GRR
and Lansing, but they ALWAYS use "double digits" for flight numbers. It's not "Comair 6-1-7-9", it's "Comair Sixty-One Seventy-Nine"... In fact, I don't remember the last time I heard someone say "Northwest 4-7-3-6" or something like that.
That being said, it's usually limited to flight numbers. A common occurrence on GRR
ATC is "expect flight level one-zero thousand one-zero minutes after departure..."
[Edited 2004-12-31 03:54:08]
[Edited 2004-12-31 03:54:42]