posted 9 years ago
For true SMS, what Tim said is correct. Several applications get around that by creating a special service that does not rely on the phone's SMS capabilities, but rather uses their own internet-based service to send stylized messages that function like SMS. They can achieve this by using a dedicated application and a web service. So to do this you would need three components:
1) a process to register users
2) a web application which accepts messages and can route them to their destination
3) a send/receive application that can be used to read and write the messages
Of course, doing it this way limits the usefulness because you can only send messages to users who registered for your service.
Some applications get around the requirements for users to register by converting styled text to images and sending those images to the target phone as MMS. Since most phones can accept MMS, this means you aren't limited by users of just your application. You might also consider having a system where, if the receiver has registered with your application then you can send them the text through a push notification to your application, and only fall back to the MMS if the receiver doesn't have your application. This could save the receiver charges for the MMS which could be significant.