That's not quite correct. You can register a number of different phone numbers. When I set mine up the first time, I was in a hotel in Los Angeles, so I had it ring the hotel room phone (I have no cell). I now have it set to my home number, and the hotel room phone number is no longer a registered phone.
However, when you register a phone, it requires you to validate that you own the phone, by ringing it up and having you enter a code, so you can't just register with a random number.
I don't mean that you're stuck with the number you originally used when registering for Google Voice. More that you have to make it forward to a number that you enter into Google Voice and it will validate that its a US number. If Bruno wants to receive calls on his Google Voice number then he'll need another US number to recieve the calls from Google Voice. It doesn't do VoIP and it won't accept non US numbers so it means you can't take calls.