We've just replied to your support ticket. However i'll paste the response here too just incase you don't receive it.
The problem is that the call forwarding rules are reset because you've changed network provider, to fix this try the following steps:
1. Reboot your phone (to give the network a refresh)
2. Ensure WiFi Calling is disabled on your phone (as this can interfere with the activation process)
3. Open your iPhone Contacts app
4. Locate the 'HulloMail Activate' contact
5. Call the activation code inside - This will 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?