"Sorry! Access Denied" error when trying to configure/customize Voice settings via ECP

I ran into an issue recently where a customer had rolled out Unified Messaging features to their users. 

Everything was working fine except for the fact that several users would receive the following error when trying to configure Voice settings using ECP:


Even though this was not the most insightful error, it did point to a permissions problem.  So next we attempted to just login via ECP with the affected user, and of course that also failed. 

As it turned out OWA was disabled for all the affected users.  We simply enabled OWA access for these users and they could now successfully customize their voice settings via ECP.