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

USA subscribers:

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 contactus@thumbtel.com