Activation is done automatically during Sign Up. 

You can also manually Activate/Deactivate within the Hullomail app:

Go to Menu/Settings -> Voicemail activation -> Activate voicemail. 

You should receive a pop-up on screen notifying you that this has been successful.

However, if the activation process fails, please try the following steps:

1. Restart your phone
2. Ensure Wi-Fi Calling is disabled (via the Phone app > Settings > Wi-Fi Calling) 
3. Open the Hullomail app
4. Goto Menu/Settings -> Voicemail activation
5. Tap 'Activate voicemail' - This will attempt to setup the call forwarding rules again

Once you've done this, give it a test by calling your personal number from another phone and leave yourself a voicemail message.

Ensure the new voicemail message appears in the app.