Using interruped uart with RTOS

I am using the EV-LM3S811 kit by luminary micro. In your sample code you disabled fifo so that you can get interrupts to get data from UART. Its working fine when I am entering data from hyperterminal(windows->communication->huperterminal) but this method fails many a times when my GSM module sends this information through UART. Without using rtos I can only perform watch on GSM module and there can be no operation in background. But using RTOS as SMS can arrive at any time and if controller is busy in other task data will be lost. I have given highest priority to UARTRx task. What else can I do? Thanx