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 subscribers:
Your Mailbox Number can be found in the Hullomail app under Menu -> 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