Next I programmed the microcontroller (AT89S52) to send the same sequence of 'at-commands' through serial port. I verified via 'hyperterminal' that the microcontroller was sending those at commands in exact order, with proper delay. This part was really tough. It took me and my group-mate several nights to accomplish.
It was the time for us to make a cable to connect mobile directly with the microcontroller, so we made a serial male-female cable with tx-rx pins in cross.
After a lot of hardware debugging we were able to successfully communicate mobile with the microcontroller.