read and send sms using AT89S52

void transmit_data(unsigned char str)		// Function to transmit data through serial port
{
	SBUF=str;		                // Store data in sbuf
	while(TI==0);	                        // Wait till data transmit
	TI=0;			  
}

void receive_data()  interrupt 4	 // Function to recieve data serialy from RS232 into microcontroller
 {
	 str=SBUF;                       // Read sbuf
	 RI=0;
	 transmit_data(str);             // Transmit to HyperTerminal
 }

First check by sending AT 
transmit_data('A');
transmit_data('T');
transmit_data(13);

In receiving part you change like below
unsigned char buffer[60];
unsigned char count;

void receive_data()  interrupt 4	 // Function to recieve data serialy from RS232 into microcontroller
 {
	 buffer[++count]=SBUF;                       // Read sbuf
	 RI=0;
 }

Then wait until it receives any feedback from GSM Module
while(count>0);
delay_ms(500);
for(i=0;i<count;i++)
{
    j=i;
   if((buffer[j]=='O')&&(buffer[++j]=='K')
   {
      LCD_data('1');
      delay_ms(300);
  }