Using Sim800c Module with arduino UNO

Hey there!
I am gonna use a Sim800c Module by an UNO Arduino... I have Tried this Module with a hyper terminal application (Putty) using TTL serial and a USB to Serial converter(FT232R USB UART) and everything was fine, but when I am gonna try this module with Arduino UNO, I got a problem. The module can get AT commands from arduino and execute the required action, for example making a call or sms etc. but I can't see the module responses from serial monitor. it seems that TX pin of module can not communicate with RX pin of Arduino.
Is that something I should do about the logic level of arduino ttl serial (if so, how can I do it?!) or the problem is something else?
I would be appreciated if you help me in this matter!
Thank you in advance