First check your voicemails are being forwarded to HulloMail. To do this open the iPhone Phone app and dial *#62# and then press the green Call button.
You should see a series of messages. The first message you should see is:
Voice Call Forwarding When Reachable Forwards to <Your Mailbox Number> Enabled.
UK & Ireland Users
Your Mailbox Number can be found in HulloMail under Menu -> Account -> Voicemail Activation
The Mailbox Number is either +1 (213) 260 9302 or +1 (213) 260 4549
If you see a different Mailbox number and you've dialled the HulloMail Activate contact, please contact firstname.lastname@example.org