GSM Modem and AT command

Hello,

i have a little probleme

I have a GSM phone connected to my pc throught an USB data cable.
When i use the hyperterminal, i can discuss with the GSM (AT command reply
"OK")
when i use my program, i receive NO reply from the GSM :(

my program work with lot of other modem without any problem !

i create the file like this :
{Create the file}
fSerial := CreateFile(
pChar(Serial),
GENERIC_READ or GENERIC_WRITE,
0, (* comm devices must be opened
w/exclusive-access *)
NIL, (* no security attrs *)
OPEN_EXISTING, (* comm devices must use
OPEN_EXISTING *)
0, (* Ansynchronous or Synchronous I/O *)
0 (* hTemplate must be NULL for comm devices *)
);


CheckError(
GetCommState(
fSerial,
MyDCB
)
);

MyDCB.BaudRate := FBaudRate;
MyDCB.Parity := NOPARITY;
MyDCB.StopBits := ONESTOPBIT;
MyDCB.ByteSize := 8;

CheckError(
SetCommState(
fSerial,
MyDCB
)
);

CommTimeouts.ReadIntervalTimeout := MAXDWORD;
CommTimeouts.ReadTotalTimeoutMultiplier := 0;
CommTimeouts.ReadTotalTimeoutConstant := 0;
CommTimeouts.WriteTotalTimeoutMultiplier := 0;
CommTimeouts.WriteTotalTimeoutConstant := 0;

CheckError(
SetCommTimeouts(
fSerial,
CommTimeouts
)
);


than you by advance for your help !

stéphane