Ralph, you are correct, in that case you could create a rotational mailbox for one of the DID's to point into the main mailbox, which has these two options enabled. but would it would still use a license.
I would therefore create a nametag hunt group with the DN of the mailbox with these fax settings on but starting with *, then point this into the voicemail. I will direct the caller into one voicemail box with fax a message facilities.