I'm skipping through the documentation and see that message.send will cause an sms to be sent to a device if it's not an iPhone. A device must have been previously added for an app for send to be possible. I can't see how to add a device with it's phone number - surely needed to be able to send sms. How is this done?
Good question! This is not well documented, but you can use the 'account/profile' endpoint to update an existing user's account with a phone number. Alternatively, you can also provide the phone number to 'user/create' when creating a new user account. In both cases the parameter (key) you'll need to set is called 'phone'.