SMS is used for various purposes these days. For example, major websites like Gmail and Facebook use SMS to improve their authentication process with multi-factor authentication and notifying users about the updates. These are one-way SMS applications since messages are sent only from these sites to the user. Two-way SMS applications are more complex than one-way ones. In two-way SMS apps, a user can initiate a conversation by sending messages, and then the application responds according to the user's commands.

They base the application on the Clickatell SMS service (not free, but cheap - pay by the message too) which includes the ability to hook into your API on a specific endpoint and relay the message data. The message can either be sent via a POST or GET and can easily be interpreted in your app extracting things like a timestamp, the number it came from and, of course, the actual text of the message. There's also a section about the "User Data Header" functionality that lets you easily split up a message for recombination on the remote device. Code is included for all examples.