Activation is done automatically during Sign Up.
You can also manually Activate/Deactivate within the HulloMail app:
Go to Menu -> Settings -> Account -> 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:
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 voicemail message then appears in the app.