They just use a rugged GSM modem such as the Wavecom units hooked to the serial port. The best way to hack them would be to open one up and remove the sim, find the number and you can then expect the others to be not hugely different, war dial the range, then guess or brute force a password. I'd guess the password is similar for all the displays the company owns. You could also get into one of the units and dump the password but that would require some knowledge.