Have you tried the 7912 with the very simple/short that I suggested?
Assuming the time syncs you can get to the WAN and as it just boots up and registers (it would seem) than the dialplan is the last obstacle.
As these cisco phones are touchy, it may be the actual service provider.
Have you ever seen a "Cannot resolve" syntax on a Cisco phone?Haven't run into it in SIP but wondering with all your SCCP work whether that has cropped up. I have tweaked the NAT/packet filtering rules on the firewall to allow 5060-5062 (UDP/TCP).This then allows your external IP address to be registered as the IP address that your phone is on.Set try turning DHCP off and also try setting the IP address of the phone to a static IP in the router.I also have a 7942G phone pre-loaded with the SIP firmware.
Soooo tempted to see whether that phone works but trying to figure this one out first.Also if you look at the link at the end of the thread Aalex guided you to you will be able to download my config file.some of it is a bit different to yours Currently using net – Internode's NTP server.If you are behind a cheap router then these options may be useful.These tell the phone to rewrite the SIP headers it sends out to the SIP proxy, to match your external address.I have also created similar rules for the stop/start media ports . If I have a look at the Status Messages I can still see "DNS Unknown Host" and "Error Updating Locale".