OK thanks for the update

The 991 mailbox on the ES is the instructional greeting of mailbox 990.

Anyway you should have the "extension" field of mailbox 0 as @F500-H as this will stop normal mailbox operation and do the "flash recall" then dial 500 and hang up performing a blind transfer to MCG 500
Also make sure the DND is OFF in mailbox 0


Regards
Carl