I dunno about your 404 issue - that's a different issue.
But, I'm sorry, I forgot to mention that you need to go into Expert Configuration mode, then go to IP Phone --> PHONE settings, and change the Primary Line value to your desired service provider (SP3 in your case). To change anything in Expert mode, you need to remove the check marks from both columns to the right of the value. Scroll down and click "Submit" after you make the change and wait for your phone to reboot. See my screenshot.