This configuration isn't allowed because it would make it impossible for users with this UM mailbox policy to play back protected ...

This configuration isn't allowed because it would make it impossible for users with this UM mailbox policy to play back protected voice mail. If the AllowSubscriberAccess property is False and the RequireProtectedPlayOnPhone property is True, the AllowPlayOnPhone property must permit the user to play voice messages over the phone from a mail client.