How to enable Unified Messaging for a shared mailbox
In some situations, you may want to have voicemails delivered to multiple employees. While there are a few options that you have available to accomplish this; if you want to accomplish this without having to pay for an additional Skype or Exchange license, you may be able to utilize an Office 365 “Shared Mailbox” and configure it with Unified Messaging (UM).
Using a shared mailbox is a great solution for those who may simply want to have a general mailbox option available from their Office365 auto attendant.
Keep in mind that this solution is a work around and is not fully support by Office 365 anymore.
To accomplish this, you'll first need to create a user account that utilizes the "Exchange Online Plan 2" license (E3 or E5 are the most common license types). You'll want to create this user account using the email address you'd like to use for the shared mailbox.
Enable UM for this new user account, by following the steps found here
You’ll then need to navigate to the Exchange Admin Center and select the "Recipients" option on the left hand side. It should take you to the active mailboxes by default.
Select the mailbox of the user account you created in step 1 and on the right hand side, select the option to "Convert to Shared Mailbox"
Delete the user account you had previously created. This will leave only the shared mailbox behind
Once UM has been enabled for the shared mailbox, you can now use the "extension" you assigned to the mailbox to use as a transfer option from your Office 365 auto attendant.
For any additional help on enabling UM for a shared mailbox, please contact support.