I have written this calculator with 2d and 3d plot function as student project. Feedback with comments is welcome. You can contact me at peirick AT gmail.com
The source code can be found on Sourceforge
Help

To input numbers use the numbers on your telephone, "#" inputs a point. To input operators or functions press "*". The position of the functions on the screen maps to the position on your mobile phone keyboard.
You can draw a graph if the formula contains an "x" or "y" and your press Calc.
To navigate in the graph press "2" and "8" for up and down and press "4" and "6" for left and right. Or press the corresponding up, left right, down key of your mobile.
To scale the graph on the y-axis press "3" for zoom out and "9" for zoom in. For zooming on the x-axis press "1" and "7". It is also possible to change the scale of the axis in the options menu.
To leave the graph window press "*" again.
The functions sin, cos, tan, ... uses as default radians as input values. You can change the trig-function mode in the options menu to radians, degrees or grad.