Ok, first off I do not know if this will work, but I think it has potential.
Why buy an expensive modem when an old phone and an old "Pocket PC" off of Ebay might be the answer, coupled with this software.
http://www.visualit.co.uk/simplesms.htmI would cable the Pocket PC to the PLC via RS232 and use the IR port between a compatible phone and the Pocket PC. You would have to build an application on the Pocket PC to glue the SMS software to the PLC.
You know if these guys are doing SMS through the phones Blue Tooth, IR PORT, or Serial connection, I wonder why it cannot be done right from the 232 port on the PLC?
Maybe a software development kit from the phone manufacturer, some minmal glue electronics, and a connector to fit the bottom of the phone would make this work. These phones today have many capabilities that we as users do not even know about. I bet there are many models out there that would allow this type of control of the phone (transfer of text via sms external to the device) via the serial connection. Goodbye expensive modem, hello $25 phone off of ebay and some investigative work.
It might take some real tinkering, and some investigative work to find a compatible phone and good documentation, but I think it would be a cool project.