I have read the other posts on this topic and have followed the advice.
My connection continues to display the error message "Registration Failed".
Having performed a full factory reset I have set the following values:
Settings -> IP Configuration -> Enable HTTP Proxy = No
In Settings -> Telephony -> Advanced Settings ->
Use Random Ports = Yes
SIP Port = 5060 - 5076
RTP Port = 5004 - 5020
In Settings -> Telephony -> Connections -> IP Connection 1
Authentication Name = 649974xxxx
Authentication Password = As supplied by Slingshot
Username = 649974xxxx
Registrar Server = akl.italk.co.nz
All other settings are as default. Linked is an image that shows these settings.
Any help would be appreciated.