I am newbie in android, I have a very simple question for android experts:- " Is it possible to call GSM AT-Commands in android just like we use in microcontroller programming through USART".
Unfortunately the response is "it depends".
If the modem is used by the Radio Interface Layer (RIL), i.e. it is in use by the system, no, unless you stop the ril daemon. You can stop it by using the "service stop rild" command from the android shell.
Stooping the RIL will prevent all functionalities depending by the phone framework to work: receiving, making and detecting calls, texts, etc.
If you stop the ril, or if the modem is in a secondary serial port (for example, it is a secondary modem), you can open the port and send AT commands as you'd do in a standard posix system. You can also use pppd, chat, etc.