Is the hardware ready? i.e have you connected the SIMCOM gsm module with the LPC 2138 controller via RS232?
Also you should take care of powering on of simcom module either by connecting one of the IO pin of the MCU to the gsm module(controlling from MCU) or you can hardwire the power on pin of the module as recommended by the simcom datasheet...
If everything is done to till above, now what you want to do???
Once the MCU is connected with the GSM module then power it ON...
In your MCU, in the RS232 port which you have connected, wait till "CALL READY" comes.
Once this is received, send whatever AT commands followed by ENTER (0x0A), to the GSM module via RS232 and you capture the response from the GSM module...Simple..