This is what our (german) "Manual of Operations for the Air Traffic Control Service" says:. .864.2: "All numbers except whole hundreds, whole thousands and combinations of thousands and whole hundreds shall be transmitted by pronouncing each figure separately.". .. .864.3: "Whole hundreds and whole thousands shall be transmitted by pronouncing each figure in the number of hundreds or thousands followed by the word HUNDRED or THOUSAND as appropiate.. .. .EXAMPLES:. .300 THREE HUNDRED. .4000 FOUR THOUSAND. .13600 ONE THREE THOUSAND SIX HUNDRED. .4300 FOURTHOUSAND THREE HUNDRED":. .. .Thatīs most probably ICAO practice, we Germans are normally great in following rules and regulations.. .Remark: we are obliged not to speak TH properly in numbers but use a T instead:Tree- Tousand for 3000, not the correct THreeTHousand. (Would rather sound like "ssree-ssousand" from the most of us). .. .Being hit by all the new RVSM levels I tend to use both pronunciations of numbers when using a "hundreds"-level, "descend Flight Level tree zero zero, tree-hundred".. .400 is common, 500 is really rare, 600 is standard for acft flying higher because Mode-C transponders shut with 600. Russian transponders donīt - we had one flying at FL610 and he showed it, american U2s or SR71s do not, they show you 600, no matter how high(er).