Refer to figures.
This question has raised concerns over the years, it seems that different CAAs use different answers.
We have added an attachment with all the information we have received on this question so far.

ICAO Doc 9432

2.4 TRANSMISSION OF NUMBERS

2.4.4 Except as specified in 2.4.5 all six digits of the numerical designator should be used to identify the transmitting channel in VHF radiotelephony communications, except in the case of both the fifth and sixth digits being zeros, in which case only the first four digits should be used.

Note 2: Caution must be exercised with respect to the indication of transmitting channels in VHF radiotelephony communications when all six digits of the numerical designator are used in airspace where communication channels are separated by 25 kHz, because on aircraft installations with a channel separation capability of 25 kHz or more, it is only possible to select the first five digits of the numerical designator on the radio management panel.

2.4.5 In airspace where all VHF voice communications channels are separated by 25 kHz or more and the use of six digits as in 2.4.4 is not substantiated by the operational requirement determined by the appropriate authorities, the first five digits of the numerical designator should be used, except in the case of both the fifth and sixth digits being zeros, in which case only the first four digits should be used.

Note 1: The following examples illustrate the application of the procedure in 2.4.5 and the associated settings of the aircraft radio management panel for communication equipment with channel separation capabilities of 25 kHz and 8.33/25 kHz: (refer to attached figure)