When can I subscribe to iCloud in Cambodia with my phone number ( 855) ?

That's entirely up to Apple. This is a user to user forum .. you are not communication directly with Apple here.

Check this link periodically > iCloud availability by country

hi parker

which service of iCloud do you want to use?

in general, almost all iCloud services just use the apple ID. so even if you have a +855 Cambodia number if you have an apple ID you can most certainly use almost all of the iCloud services.

which one(s) do you want to use?

thanks