Hi Kan,
we use below REST API to aceess mailbox details for normal voice mail user and for that we need credential of that voice mail user, its not accessible using admin account and there is no any settings for that.
https://10.102.90.11/vmrest/mailbox Admin can access below url:
1) https://X.X.X.X/vmrest/mailboxstores
you can get mailboxstoredetails with mailboxstoreobjectid "20f8e46e-9604-4cdc-bea9-ef9871446a0f"
2) https://X.X.X.X/vmrest/mailboxstores/20f8e46e-9604-4cdc-bea9-ef9871446a0f/mailboxes
you can get total mailboxes.
Thanks
Anil Singh