You might want to try this...
Turn on the Call-Back feature on Linux, effectivly turning you box into an ISP, make sure you use you Mobile for the connection....
SMS yourself using another mobile, with the linux box listening,make a script that echos all incoming data, de-crypt the signal from the SMS, read the headers (try ROT13 encryption), and you should have enough information to send back SMS messages...without another blood sucking third party.....
.:.LJCSystems.:.