With regards entry of the the flight number this is what my Smiths /Thales 'New FM2+ Pilots Guide has to say on the matter;
Section II-Flight Phases Procedures
As per page 10 of the thales FMS 2 info booklet......................'this information is sent to various applications (as available) e.g ATSU, ACARS Flight Recorder; it must therefore be entered exactly as written in the ATC flight plan.'