Can you please try the following to see if it gets resolved:
1. Delete the HulloMail app off your phone
2. Reboot the phone
3. Reinstall HulloMail from the GooglePlay store
4. Open HulloMail and sign in
5. Using another phone, please call your personal phone and reject the call to HulloMail voicemail service
After you hang up, do you now get a notification come through on the HulloMail app?