[crumpled]

"Yes, but Google Voice is only available in the US." It would be more true to say: Google Voice only provisions US phone numbers. I have used my google voice number to make and receive calls and SMS messages from other countries. You CAN use it worldwide, it's only TCP/IP data after all. Unfortunately, you can't count on MMS working if you send to a non-US carrier. But MMS is only good because of its de facto status, it's not the best option for any of its use cases.

[dheera]

At least now, Google only permits you to get a Voice number if you can prove you have another US number to forward to (they dial you to verify). Although I do have a US number, that's only because I happen to be working in the US right now; if I weren't I probably wouldn't.

Also a lot of apps in various other countries (notably China, because I'm familiar with it) do registrations via SMS, in which you need a +86 number in order to register, period. Smaller companies don't have the infrastructure to send international SMS verification numbers. Larger Chinese companies (e.g. Weibo, Wechat) permit registration with a US number, but they're larger companies who have servers in the US and infrastructure with US SMS gateways. So running around the world with a US number isn't always practical. If SMS is dethroned as a "de facto" communication method that everyone is expected to have, this might change. Other places besides China may also have a similar situation.