serial port, modem not responding pls help...

I create a simple program using Win32 API that will send AT commands and receive the response from the modem.
The problem is that the modem does not seem to response.
I try connecting 2 PC with serial cable and running hyperterminal on other pc, the program works fine. The modem works fine also, hyperterminal connection and testing the modem run fine.

I try setting any dcb config with no avail, what's the difference with communicating with your modem and to other pc? For your information my modem is internal and I can't test with external modem (don't have one) and the program runs on single thread (try 2 threads also the same).
What configuration that I forget to set up for my modem that is different with standard serial comm? Any idea?