Here is an audio/video file with transcript about how the numbers must be pronounced according to the ICAO (International Civilian Aviation Organization) standard. This is how aircrew members, and air traffic controllers should transmit the numbers.
CAUTION – There is not any exception for FL 100, and FL 200 according to the ICAO DOC 9432 Radiotelephony Manual, page 19, chapter 2.4.2, as it is pronounced « FLIGHT LEVEL ONE-ZERO-ZERO », and « FLIGHT LEVEL TWO-ZERO-ZERO ».
However, « Flight level one hundred » follows the French DGAC and the British CAA patterns.
Click on this video: