3rd party sip devices need to have the extension number and username the same or they won't register.
Check your whitelist setting for that extension.
You haven't configured a DHCP option the polycom is looking at by accident have you?
I've never done a IP7000 but a few IP6000 and IP5000 which are similar.
Did you configure the polycom via the phone or via the web interface?
I've found that with V4.x and higher polycom the web interface is good.
Have a look in 5.2.xx (i can't remember exactly) in DM it's called ACL or access, it will give a reason for the sip fail (username/password error, etc)