can't send command to controller with a siemmen mod sim300

[juanramos100]

Hi,
I have serial gsm modem connected to a microcontroller aplication, I call the modem from a remote computer using hyperterminal, the modem answers and connect but when I try to send command to the microcontroller , using hyperterminal, the modem dosn't pass the command to the microcontroller.
I'm using a modem from linkto model LT60-340, it use siemmens module 300.
when I use other model like LT60-56 it works. I don't know what to change in the LT60-340 model to make it work.

Please help me.

Thanks.