Arduino Mega and SIM808 response problem

Hi guys
im new here , i bought sim808 and its work well for first , then the serial port shows no response at all even for AT Command only ! i thought the problem with arduino or sim808 module , then i tried ATD with my number and its call me and i can hang up by ATH !! so its work.
for that i got some application test the serial port and its response with special characters and unreadable thing as the picture.

i tried change baud rate and even use my desktop computer and laptop the same result.
if anyone has any idea or had the same problem advice me please.
best regards

no one has the same problem ?