It sounds as though you're not activated yet which means your call forwarding isn't setup to divert calls to us yet, to fix this try the following steps:
1. Reboot your phone
2. Ensure WiFi Calling is disabled on your phone
3. Open your iPhone Contacts app
4. Locate the 'HulloMail Activate' contact
5. Call the activation code inside - This will try setup the call forwarding rules again
Now to test, use another phone and call your personal phone, then reject that call and ensure it gets forwarded to the HulloMail Voicemail deposit service.
Does it now work?