Your getting the idea. Your right that VP needs to maintain contact with the server. The server actually acts as directory operator. It is easier for people to rememeber phone number than IP address, also public IP address can be changed from time to time making it harder for deafies to call another VP if using IP address method. That is the purpose of the server, simplify the contact informatin for everybody. VP needs to have port #1720 to be open all the time. WHenever your friends call in, server gets in your VP using that port and signal incoming calls. If you block it, server have no way to alert your VP that somebody is calling you.
2 routers on same public IP address won't work, and cause collision... Really it is like trying to have two master control a dog, then that poor dog gets confused.
DHB