You need a US number and a US IP address to register but thats the easy part. Once you've registered then you don't need either but there is another issue.

If you're not in the US then Google Voice may not work properly for you. It doesn't do VoIP. You're actually calling an access number which then connects you to your destination. Incoming calls are also via an access number. I'm unsure what happens if you're in Canada though. It doesn't work at all here in the UK.

All you'll get is SMS and voicemail. No MMS.