This is just a theory, because I have yet to personal try this myself.
So when you sign up for a Google Voice Number, you'll need to verify with a valid number. (This much you already know).
You can verify your number one of two way:
1. Google's calls the number you've provided
2. Google's texts the number you've provided
Giving you the code you need to verify your account.
There are many apps you can get for smart phones that'll provide you with a free number. (Like free texting apps)
I know talkatone is one of them.
You might want to try getting on of these free texting numbers, and have Google Voice text you the code via the free app number.
Like I said this is just a theory, and I haven't had a chance to test it out yet. But I hope this helps, and feel free to let me know if it works out.