Only way I can think you may be able to achieve this is use the operator extn within the mailbox.
From mitel help:-
Enter a location to which calls are to be directed when a caller presses"0" while listening to a greeting and recording prompt for this mailbox. A location can be any valid destination from 1 to 20 digits including extensions, external telephone numbers, hunt group access codes and system abbreviated speed dialing numbers. The default sends calls to Mailbox 0, the mailbox usually reserved for the company-wide operator. Specifying a destination other than the company-wide operator is useful in multi-tenant applications where one system services two or more businesses. With flexible Dial "0" destinations, each business can redirect calls to separate locations. This feature also accommodates users who want callers to transfer to a departmental secretary, rather than a company-wide operator