First check your voicemails are being forwarded to HulloMail. To do this dial *#62# on your iPhone.
You should see a series of messages. The first message you should see: 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 +12132609302
If you see a different Mailbox number and you've dialled the HulloMail Activate contact please contact firstname.lastname@example.org