You don't have a true gateway you have an SMS application. As you know, you can send an outbound SMS message as long as you know the number and the provider. Inbound SMS isn't provided by the Cell phone providers to the public. A gateway will handle both inbound and outbound traffic for you but it isn't free. An example is Clickatell.. You can do a search and find others. If you were looking for something for free, it's probably possible to use Twitter as a gateway but it would take a bit of research and your users would probably have to be signed up with Twitter for it to work.