Network stack with USB modem

Hi,

I'm working on application that uses a USB HUAWEI MU709 modem with STM32F407. First off all, i used the usb stack from keil. I follow the USB PL2303 example. The usb communication works fine. Then, to have a PPP connexion, i used the network stack from kiel.
There is an example with Custom modem, i flowed it to make my own.
The modem driver control block is as follows:
DRIVER_MODEM Driver_MODEM;
DRIVER_MODEM Driver_MODEM = { Initialize, Uninitialize, Listen, Dial, Hangup, IsOnline, Notify
};

The problem that i have is with Notify callback. When i send my AT commands to the modem, i always have a timeout. But, the modem is responding with the expected responses.
Can you please tell what would be the problem?

Best regards,