Hi Thomas,
The second UART (UART1) is the same as the one that connects to the 6 FTDI compatible pins next to the ethernet jack. You might be able to use the IO2 and IO3 pins for interrupts and use the second UART (UART1) on the other pins (the FTDI ones), just remember that it's 3.3 V only.
SoftwareSerial as used on Arduino isn't really available on the Galileo boards. If it works on IO2 and IO3, it's gotta be some hacking on the code side that simply hooks it up to the second UART (UART1) and sets the Muxes.
You could maybe also use a different pin for your CALL signal from the GSM module and do simple polling, but it's not as clean.
Hope it helps!
/Thomas