I am having problems getting my Siemens Gigaset A580 IP phone to register my iTalk connection.  I am successfully using several iTalk accounts with other devices including the Linkysy SPA942.  I am confident the account, network connection and router settings are not the source of my issue.

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 ->
I have:
Use Random Ports = Yes
SIP Port = 5060 - 5076
RTP Port = 5004 - 5020

In Settings -> Telephony -> Connections -> IP Connection 1
I have:
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.

http://www.paddletots.co.nz/tmp/A580_Settings.jpg
 
Any help would be appreciated.