If you have GV configured to pass-through caller-ID of the original caller (so you can see who is calling you), then you are charged Verizon minutes when they call you since Verizon sees the caller-ID as being the persons real number.
If you have GV configured to not pass-through caller-ID, and add your normal GV number to your favs, all incoming calls are free. Unfortunately you will lose all caller-ID this way as all incoming calls will look like they are your GV number calling you. You can use call-presentation, but I find that clumsy.
Also, remember that the outgoing number that the official GV app in Android uses to dial-out can change on you (and it is NOT the same as your GV number!). I believe it is related to relatively large geographic areas, but be sure to memorize it and keep an eye on it to make sure it doesn't change on you and cause you a surprise bill. I live in the SF Bay Area, and it's been the same in the South and East Bay, across about 60-70 miles. I've heard it's different elsewhere, but haven't traveled since I went all-in on GV.