Whichever extension you will be configuring as your primary (200 would be assumed here).
You set the max ringtime to 60 secs, and calls still do not go through? If so, set up two SIP phones on pbxes.org; and try calling between them to make sure you are configured correctly there.
I guess it's either "Works 100%" or "Fails 100%" with the automated setup
Also, pbxes.org has a forum, where the owner and several staff are on there consistently
I think the only way to forward your cell number to your GV number is to contact VZW, and set up the forwarding. In fact, I am about 99% sure this is the ONLY way to do it.
The best way is to just hand out your GV number instead of your actual cell number (Or home, etc). This way, you can let GV do what it does best: Make people able to hunt you down, with a single phone number.
What two boxes?
Gizmo5 has a already-built interface to Google Voice...