The FAA’s ATC orders (section 2-4-20) say that the ‘group form’ should be used by controllers when talking to any aircraft with an FAA authorized callsign:
Air carrier and other civil aircraft having FAA authorized call signs. State the call sign followed by the flight number in group form.
NOTE- “Group form” is the pronunciation of a series of numbers as the whole number, or pairs of numbers they represent rather than pronouncing each separate digit. The use of group form may, however, be negated by four-digit identifiers or the placement of zeros in the identifier.
EXAMPLEâˆ’ “American Fiftyâˆ’Two.” “Delta One Hundred.” “Eastern Metro One Ten.” “General Motors Thirty Fifteen.” “United One Zero One.” “Delta Zero One Zero.” “TWA Ten Zero Four.”
In Europe and throughout the rest of the world you will hear the group form pronounced differently then in it is in the US due to their different regulations. The US’ Group form is much shorter to say, taking up less radio traffic.