All you should need for Gizmo5 n the N900 is your User name and acount password. Just leave the rest of the SIP settings at their defaults. The user name you need use if the 1747... number that was assigned to your account.

1747......@proxy01.sipphone.com

And it should sign in without any problem (assuming you have a 'Net connection running.

To make a SIP call, just change the Call Type on the Dial Pain to your SIP account. and dial.