Print output from serial device connected on COM port using serial module

I am trying to use the following modules in p3

controlP5 (for GUI)
serial (to communicate with a device on COM port)

... the device is a GSM SIM900 module that can understand AT commands. when I create a session in PUTTY for COM port, for every AT command I type and press ENTER, I get an acknowledgement. For e.g.

AT
OK

where AT was the command I typed and OK was the acknowledgement I got.

In p3, I made a GUI that contains 2 textboxes and a button.

I also created a serial handle,

mpo = new Serial(this,Serial.list()[0],9600);

mpo is global object of class Serial

On clicking the button, the content in the textboxes is processed and if a condition is met, then some functions in serial module are called.

mpo.clear();  
mpo.write("AT\n");

... I also have this function,

void serialEvent(Serial mp){  
ins = mp.readString();  
print(ins);  
}  

where ins is a global String object. This function, I suppose, is called whenever I send commands to module and not when the module acknowledges. So, it prints AT, but doesn't print OK... Is there any way this problem can be solved?