Sounds like Google are SMSing you from their own number, on behalf of Allo users -- at least for iPhone. If you click on the iPhone tab of that page, you see t

Sounds like Google are SMSing you from their own number, on behalf of Allo users -- at least for iPhone. If you click on the iPhone tab of that page, you see the following: Respond to a message without downloading Allo When you receive a message from an Allo user, you have a few options: Reply with a text message to continue chatting with the Allo user. To stop receiving messages from Allo users, text "STOP."