US-only, of course, but:
On the receiving end, when you get a text message from Gmail on your phone, it will come from a number in the 406 area code. You can reply to this text on your phone just like you’d reply to any other text. The reply gets routed back to our Gmail servers and shows up in your friend’s Gmail chat window. Each of your friends’ messages will come from a different 406 number so you can reply to any message and it will get back to the right person. Messages from the same person will always come from the same number, so you can even bookmark it in your phone.