The Horn / Whistle Button and the F2 both control the same sound but in different ways.
Using the Button labelled "Horn / Whistle" is momentary.
Using the Button F2 (the Actual 2 button) is on / off.
For NCE mobile decoders only, the corresponding CV information translates into:
CV36 F2 default value is 8 output 4 / violet
CV123 EFX for F2 output 4 / violet default value is 0 - standard on / off
The ability to remap the horn / whistle button to a different function on the cab was removed in 6/2007 of the command station firmware