Hi G0rdon,
Depending on which shield you have, you might be able to simply use the regular serial port on Arduino pin 0 and 1 - called Serial1 in the Arduino IDE and get rid of all the softserial code.
Contrary to the regular Arduinos, this serial port just connects to the quark chip (the linux system and your sketch code). You will still have the other serial connection that you can use to send back messages to your computer and read in the terminal of the Arduino IDE - simply called Serial in the IDE.
Using AT commands, you should be able to get both sender number and message without any problems... but you will probably need to look in the data sheet for your GPRS shield/module to find the exact AT commands, especially to find the sequence for initializing the module and maybe entering SIM PIN, etc. if needed, to be allowed to communicate with the module at all.
If you want to play with AT commands from your computer, it should be possible to make a simple Arduino sketch for the Galileo, that passes all data from one serial port to the other.
Hope it helps.
/Thomas