programming

int speaker = 9;
int frequency1 = 31000;
int frequency2 = 41000;
int frequency3 = 51000;
int frequency4 = 61000;
int frequency5 = 2000;
unsigned char Buff[250];
unsigned char BuffIndex ;
void setup() {
  // put your setup code here, to run once:
pinMode(speaker,OUTPUT);
Serial.begin(9600);
delay(3000);
Serial.print("ATS0=2\r\n");  
delay (3000);
Serial.print("AT+DDET=1\r\n");
delay(3000);
memset(Buff,'\0',250); //initialize the string
BuffIndex = 5 ;
}

void loop() 
{
  // put your main code here, to run repeatedly:
    while(1)
{
  if(Serial.available()>0)
  {
    Buff[BuffIndex]= Serial.read();
    
    if( (Buff[BuffIndex-5]=='D')&&(Buff[BuffIndex-4]=='T')&&(Buff[BuffIndex-3]=='M')&&(Buff[BuffIndex-2]=='F')&&(Buff[BuffIndex-1]==':')&&(Buff[BuffIndeX]=='0'))
    {
      tone(speaker , frequency1);
      }
      
    if( (Buff[BuffIndex-5]=='D')&&(Buff[BuffIndex-4]=='T')&&(Buff[BuffIndex-3]=='M')&&(Buff[BuffIndex-2]=='F')&&(Buff[BuffIndex-1]==':')&&(Buff[BuffIndeX]=='1'))
    {
      tone(speaker , frequency2);
      }
      
    if( (Buff[BuffIndex-5]=='D')&&(Buff[BuffIndex-4]=='T')&&(Buff[BuffIndex-3]=='M')&&(Buff[BuffIndex-2]=='F')&&(Buff[BuffIndex-1]==':')&&(Buff[BuffIndeX]=='2'))
    {
      tone(speaker , frequency3);
      }
     
    if( (Buff[BuffIndex-5]=='D')&&(Buff[BuffIndex-4]=='T')&&(Buff[BuffIndex-3]=='M')&&(Buff[BuffIndex-2]=='F')&&(Buff[BuffIndex-1]==':')&&(Buff[BuffIndeX]=='3'))
    {
      tone(speaker , frequency4);
      }
      
    if( (Buff[BuffIndex-5]=='D')&&(Buff[BuffIndex-4]=='T')&&(Buff[BuffIndex-3]=='M')&&(Buff[BuffIndex-2]=='F')&&(Buff[BuffIndex-1]==':')&&(Buff[BuffIndeX]=='4'))
    {
      tone(speaker , frequency5);
      }
    
      BuffIndex++
      {
      if(BuffIndex>250)
       BuffIndex = 5;
      }
  }
  
}
}

i dont know what the error is. i have tried compiling it.

BuffIndeX ( note capital X ) is not the same as BuffIndex, in a bunch of places.

 tone(speaker , frequency5);
      }
   
      BuffIndex++  // missing semi colon
      {
      if(BuffIndex>250)

Without looking at anything else:

int frequency2 = 41000;
int frequency3 = 51000;
int frequency4 = 61000;

These values are all outside the size that an int can hold (-32768 to +32767).

This looks a little odd

BuffIndex++
      {   //<--- thinking that's in the wrong place
      if(BuffIndex>250)
       BuffIndex = 5;
      }
BuffIndex++

Something missing here