You can start with wordpress EasySMS plugin. Most of the code would be same except the way it connect to sms gateway. The main features we are looking for are as follows.
1. People should able to register (subscribe feature in EasySMS plugin) and verify their phone no.
2. All verified user can send free sms to india.
3. Option to prefix / suffix a sms that one user send.
4. Way to group users.
5. Site admin should able to send bulk sms to groups.
6. A send sms widget with embed code so that send sms widget can be placed on other sites.
7. Send widget sms should ask user to register before they can send free sms. This workflow can be done using jquery dialog box.
8. Way to add sms credit to user. User would be able to send sms if they have sms credit.