How do you send AT commands using Dynamic C through Rabbit 2000 controller connected to a GSM modem?

Question

First, I am working on a project that opens an electric door when certain cell phones call the GSM modem's SIM card's number. But, as a starting point I want to send some AT commands to the modem, then from there I can continue. Here is the code I wrote so far:

#define CINBUFSIZE 255
#define COUTBUFSIZE 255
#define TIMEOUT 500
char modembuf[255];

void INIT(void){
    serCopen(115200);
   serCflowcontrolOff();
   serCparity(PARAM_NOPARITY);
   serCwrFlush();
   serCrdFlush();
}

void clrbuf(void){
    serCwrFlush();
   serCrdFlush();
   memset(modembuf, 0x00, 255);
}

int fnMsDelay(unsigned long delay){
    unsigned long u10;
   u10 = MS_TIMER;
   while((MS_TIMER-u10)<delay);
   return 0;
}

void GSM_INIT(void){
    int looping;
   looping = 0;
   puts("\nGSM_INIT");
   tryagain:
    clrbuf();
      serCputs("AT\r");//waking modem from sleep mode
      fnMsDelay(TIMEOUT);
      serCread(modembuf, 255, TIMEOUT);
      printf("\nReady to accept commands: %s", modembuf);
      if(strstr(modembuf, "OK")){
        clrbuf();
          serCputs("ATDT*some_number*");//calls *some number*
          fnMsDelay(TIMEOUT);
          serCread(modembuf, 255, TIMEOUT);
          printf("\nCalling 0522386652, status: %s", modembuf);
          if(strstr(modembuf,"OK")){
             printf("\nGSM_INIT OK");
             BitWrPortI(PADR, &PADRShadow, 0, 0);
             clrbuf();
             return;
          }
        }
    looping++;
   printf("\nLooping: %d\n", looping);
   if(looping==20){
    printf("\nGSM_INIT FAILED");
      return;
   }
   goto tryagain;
    serCclose();
}

void main(void){
    INIT();
    GSM_INIT();
}

However, every time I run the above code the program fails to send any AT commands. Why? And how do I fix this problem?