Thanks for your reply/suggestion: I already tried that setting.
It was the first setting I played around with, after we noticed that fax-receiving does not work.
Default, After 1 CED tone, After 2 CED tone does not work with Mailbox enabled with FAX only - I did not try After 3 CED tone, btw: switching/changing the Switch to T.38 is not being saved at all, it shows the checkmark if I click save button, but after refreshing that page it is still on "Default".
so I canged/tried "global setting":
I changed the Switch to T.38 at Administrator level / General/System - there the "save" works, but for Mailbox in FAX only mode it does not make any difference. Only after Mailbox enabled to "Enabled" the Extension will receive fax-messages/transmissions.
But then callers can also leave voice-messages, that is unwanted at our "fax only" Extensions.