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:

1. Reboot your phone
2. Ensure WiFi Calling is disabled on your phone
3. Open the HulloMail app
4. Goto Menu -> Settings -> Account -> 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 voicemail message then appears in the app.