From what I have gathered, it is possible to forward your cell number to GV (which will in turn forward to all phones - including your cell), but it is not advised. The ring would be very long and it would be like 9 rings before you even hear it or something along those lines. There are other problems I have read about, such as problems where SMS or text messages never make it through the forwarding process. (Might want to read through this as well before you try it:
Top 10 Reasons NOT to forward a phone TO Google Voice - Google Voice Help ) You might take this opportunity to just start gradually shifting everyone you know over to your GV number so everyone starts calling it instead, and you can make sure GV always forwards to your cell/etc other numbers in the future (this is the route I am working on taking). Also, there IS a way to port your current cell number over to become your new GV number. If you do that, you will be able to set up GV to forward to all your phones and everyone can call your regular number. Of course, I have heard that if you port your number from an active verizon acct it can cancel your contract and leave you with an early termination fee (if that applies).
About the WiFi calling: As long as you are using a VOIP connection going over WiFi it will be as if you are still in the states (but don't have cell service). You can test this out by going on your phone and setting it to "Airplane Mode". After that, go turn on WiFi connection and make a call. You will see that Airplane mode stays enabled even if you just turn on WiFi. This way you have completely turned off your cell service and you can ensure that you are making a call via WiFi connection instead of 3G or over your cell service. When you have your cell service turned off, you can do the test as if you are in a different country without cell service. All you will need is a WiFi connection. Don't worry about GPS, the phone will not check your GPS to ensure you are in the states. To put this short, there are not any extra steps when you are out of the country (as far as I know), and it should work fine. I
will say that I have never made a WiFi call out of the states but I have read many posts about VOIP on Android and I have come to this conclusion.