Hi Sandeep,
I guess the max232 IC circuit between the 8051 and the SIM900 is working perfectly.
I also had the same problem, it was due to some power supply connection issue when connecting with the microcontroller board, the SIM900 will respond with AT commands but not able to make a call or send message.
Please post here the exact circuit you are using right now.
First try to fix this issue and then we will look into the LCD problem.