We don't have specific recommendations on ITSP, but it should work with most SIP based providers. As far as configuration, the easiest way to do this is to use the spa9000 wizard.
For your spa50x phone, it may be that the firmware is outdated, by running the wizard, it will check if the firmware is out of date and update it as needed.
The spa9000 wizard should also automatically find the spa400 and configure it.
One recent issue that was found is that if your spa400 is going through reboots, change the ip address of it from static to discover after the wizard has completed it's changes (as the wizard sets the spa9000 and spa400 to static).