KalSMS is an Android app that acts as a SMS and MMS gateway. 
For more information, see http://youngj.github.com/KalSMS/

Features
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*   Can be deployed almost anywhere in the world
*   Designed to allow non-technical users to do all setup/maintenance 
    of deployed phones
*   Can send outgoing SMS in excess of Android's limit of 100 messages 
    per app per hour by installing expansion packs
*   Many different phones running KalSMS can be connected to a single server 
    (each phone could forward messages for a different mobile network)
*   Authenticates with password shared between phone and server
*   Retries forwarding messages after a delay if there is an error 
    (e.g. due to temporarily broken internet or GSM connection)
*   Notifies the server of the status of outgoing messages
*   Forwards incoming MMS messages (pictures/video/audio)
*   Shows log messages to user to facilitate troubleshooting
*   Can prevent incoming messages from being stored in Messaging inbox
*   Can forward messages already received in Messaging inbox
    
License
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The code is released under the MIT license; see LICENSE