There is already a nice open-source implementation of this application on android: SMSSync

An Android application that sends and receives SMS and update to sms-db.

• API URL for sms_received - Received SMS will be POST-ed to this API
• API URL for pending - A URL to fetch SMS that are waiting to be sent
• API URL for sent - When SMS is sent, POSTs the id to this API
• API Key - Every SMS terminal has to have its specific api key. It has to be passed whenever an API request is made.
• Crash report URL - An acralyzer application URL to POST submit report to.
• Crash report username
• Crash report password

There are 3 entities interacting throughout this application.

• SMSDB - Stores SMS that are received and to be sent
• SQLite - Stores SMS that are being sent or received. Whenever Mobile network or Internet failure occurs, it will retry the stored SMS.
• Periodic Service:
  • Send - Fetch pending SMS from SMSDB and save to DB
  • Send - Get pending SMS from DB and send. Delivery report will set the SMS state to sent in DB.
  • Send - Get pending SMS which has been successfully delivered from DB and post to SMSDB
  • Receive - Triggered by android.provider.telephony.SMS_RECEIVED. Saves received SMS to DB.
  • Receive - Get received SMS from DB and posts to SMSDB
  • Cleanup - Remove SMS that are marked as synced

Following types of SMS might occur in the SQLite Database:

+----+------+--------------+---------------------+----------+--------+--------+
| id | body |    phone     |     created_at      |  status  | sms_id | synced |
+----+------+--------------+---------------------+----------+--------+--------+
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 | to_send  |   71   |   0    |
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 | sending  |   71   |   0    |
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 |   sent   |   71   |   0    |
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 |   sent   |   71   |   1    |
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 | send_fail|   71   |   0    |
| 1  | abc  | +97698111111 | 2014-02-06 08:24:00 | send_fail|   71   |   1    |
| 2  | foo  | +97688776655 | 2014-02-07 09:00:00 | received |  null  |   0    |
| 2  | foo  | +97688776655 | 2014-02-07 09:00:00 | received |   78   |   1    |
+----+------+--------------+---------------------+----------+--------+--------+