Number porting is used to keep a number you want. If you want to port your existing number to Airvoice, you have to do so at the point of activation and not after. When you port a number, it terminates service with the carrier that number was with and switches it to a new provider. As for not being able to port a number into Google Voice? It happens with some numbers on some exchanges.
https://en.wikipedia.org/wiki/Local_number_portabilityAnd yes, Airvoice will
always charge you for any airtime spent on their phone network, even calling Google Voice. Using GV is not some magical way to get free phone calls through traditional phone service, that's not how it works. All it is is a VoIP based incoming call forwarding service with the ability to call out as well
without using your phone and its data/minutes by using a software phone to make calls over the internet off of an already paid for WiFi data connection (amongst other methods). If you use your cell phone's phone number for any part of connection or termination with Google Voice calls, you will get charged for the minutes used.
http://google.about.com/od/socialtoolsfromgoogle/p/what_is_google_voice.htmBefore spending any more money, it might be best to tell us what you're actually trying to accomplish, work out what your actual calling needs are, and you should take more time to familiarize yourself with the technology you're trying to use to accomplish the tasks you're after. Also,
don't be afraid to pay for what you need, especially if you're technologically challenged. Free sounds all nice and awesome, but you get what you pay for and it rarely ever is as awesome as it sounds.