Ah! I thought this might be the answer, after I tried installing the T-Mobile Visual VM app but found it does not work with my service (Mint) and later read here that that app is only for older phones, as Android now includes visual VM without need for a specific app. But after following the above instructions, restarting my BBB100-7, and sending a test VM, I still get an error message "Can't activate visual voicemail".
Is there any way to get Visual VM to work with Mint? (The Mint rep has no idea.)