Hola
vamos a ver el interés que tienes en este programa :-)

te voy a poner el código fuente del programa en java
si tienes interés seras capaz de compilarlo y crear el jar
el proyecto consta de dos ficheros
---------------ConvierteLetrasNumeros.java-----------------------
package org.riky.conv;

import javax.microedition.midlet.*;
import javax.microedition.lcdui.*;

/**
* <p>Title: </p>
*
* <p>Description: </p>
*
* <p>Copyright: Copyright (c) 2008</p>
*
* <p>Company: </p>
*
* @author Riky
* @version 1.0
*/
public class ConvierteLetrasNumeros extends MIDlet {
static ConvierteLetrasNumeros instance;
PantallaConversion displayable = new PantallaConversion();
public ConvierteLetrasNumeros() {
instance = this;
}

public void startApp() {
Display.getDisplay(this).setCurrent(displayable);
}

public void pauseApp() {
}

public void destroyApp(boolean unconditional) {
}

public static void quitApp() {
instance.destroyApp(true);
instance.notifyDestroyed();
instance = null;
}

}

---------------ConvierteLetrasNumeros.java-----------------------

---------------PantallaConversion.java----------------------------
package org.riky.conv;

import javax.microedition.lcdui.*;

/**
* <p>Title: </p>
*
* <p>Description: </p>
*
* <p>Copyright: Copyright (c) 2008</p>
*
* <p>Company: </p>
*
* @author Riky
* @version 1.0
*/
public class PantallaConversion extends Form implements CommandListener {
TextField tf=new TextField("Introduce Letras","",80,TextField.ANY);
TextField ta=new TextField("Resultado","",200,TextField.ANY);
public PantallaConversion() {
super("Convierte Letras en Numeros");
try {
jbInit();
}
catch(Exception e) {
e.printStackTrace();
}
}

private void jbInit() throws Exception {
this.append(tf);
this.append(ta);
// Set up this Displayable to listen to command events
setCommandListener(this);
// add the Exit command
addCommand(new Command("Salir", Command.EXIT, 1));
addCommand(new Command("Convertir", Command.OK, 1));
}

public void commandAction(Command command, Displayable displayable) {
if (command.getCommandType() == Command.EXIT) {
// stop the MIDlet
ConvierteLetrasNumeros.quitApp();
}
if (command.getCommandType() == Command.OK) {
convierte();
}

}

/**
* convierte
*/
private void convierte() {
String cad=tf.getString().toLowerCase();
String tmp="";
for(int n=0;n<cad.length();n++){
tmp+=letr2num(cad.charAt(n));
}
ta.setString(tmp);
}
private String letr2num(char c){
int num=0;
String ret=" ";
switch (c){
case 'a':
num = 98;
break;
case 'b':
num = 22;
break;
case 'c':
num = 93;
break;
case 'd':
num = 432;
break;
case 'e':
num = 199;
break;
case 'f':
num = 2;
break;
case 'g':
num = 6;
break;
case 'h':
num = 88;
break;
case 'i':
num = 99;
break;
case 'j':
num = 174;
break;
case 'k':
num = 101;
break;
case 'l':
num = 123;
break;
case 'm':
num = 31;
break;
case 'n':
num = 444;
break;
case 'ñ':
num = 56;
break;
case 'o':
num = 45;
break;
case 'p':
num = 789;
break;
case 'q':
num = 100;
break;
case 'r':
num = 999;
break;
case 's':
num = 457;
break;
case 't':
num = 11;
break;
case 'u':
num = 4;
break;
case 'v':
num = 9;
break;
case 'w':
num = 89;
break;
case 'x':
num = 56;
break;
case 'y':
num = 1;
break;
case 'z':
num = 7;
break;
case ' ':
ret=" ";
break;
default:
ret = " El alfabeto tiene 27 letras.\n";
}
if(num!=0)
ret+=num;
return ret;
}

}

---------------PantallaConversion.java----------------------------

El programa funciona y compila sin errores.

venga saludos.