Account Managers (formerly known as Portal Users) can manage their Business Cloud Auto-Attendants in the end user's Voice Portal, but you'll need to give them permission first.
Go to Account > Settings: General Settings.
Scroll past the account information until you get to Voice Portal Permissions.
To allow Account Managers to access their Auto-Attendants in the end user portal, check Allow Editing.
Scroll to the bottom of the page and click [Save].
Info
This setting applies only to Account Managers on Business Cloud Communications. Platform Users cannot access Auto-Attendants in the Voice Portal. See also Voice Portal Permissions.