C / C++ / MFC

SetConsoleActiveScreenBuffer(hScrBuf);

Or you should use

hSrcBuf = GetStdHandle(STD_OUTPUT_HANDLE);

LONG   res;
LPVARSTRING p_varString=NULL;
size_t size=sizeof(VARSTRING)+1024;
HANDLE hCOM;

for( ; ; ){
  if((p_varString=(LPVARSTRING)malloc(size))==NULL)return FALSE;
  p_varString->dwTotalSize=size;

  if(lineGetID(0,0,hCall,LINECALLSELECT_CALL,p_varString,"comm/datamodem")!=0){
    free(p_varString);
    return FALSE;
  }
  else if(p_varString->dwNeededSize>size){
    size=p_varString->dwNeededSize;
    free(p_varString);
  }
  else break;
}
hCOM=*((LPHANDLE)((LPBYTE)p_varString+p_varString->dwStringOffset));
free(p_varString);

1. check WriteFile works with a handle obtained with CreateFile --just get the handle without establishing any connection before.
   
2. Make sure the modem is behaving properly --by using some TAPI program like HyperTerminal.
   
3. Try to determine if the handle is valid by calling other functions different from WriteFile() and checking the result (with GetFileType, for instance).
   
4. Make sure the modem port settings are compatible with the requirements of the modem (port speed, parity, stop bits, control flow).
   
5. See what happens with different modems/drivers.
   
6. See what happens with different OSs/machines.
   

• You've opened the port with FILE_SHARE_READ|FILE_SHARE_WRITE: Microsoft docs say dwShareMode must be zero for communications resources.
  
• The piece of code

  if(bResult == 0)
  {
    MessageBox(NULL, "WriteFile() failed", NULL, 0);
    //dwLastError = GetLastError();
    sprintf(str1, "Return value of GetLastError() = %d", GetLastError());
    ...

  should be written

  if(bResult == 0)
  {
    DWORD dwLastError = GetLastError();
    MessageBox(NULL, "WriteFile() failed", NULL, 0);
    //dwLastError = GetLastError();
    sprintf(str1, "Return value of GetLastError() = %d", (int)GetLastError());
    ...

  to avoid the possibility of MessageBox altering the error code.