Telephone using c++

#include <iostream.h>
#include <ctype.h>
#include <stdlib.h>

typedef int Bool;
const Bool TRUE = 1;
const Bool FALSE = 0;

void readLetter(char&);
int computeCorrespondingDigit(char);
Bool doOneLetter(); //returns flag saying whether you should quit

void main () {
  Bool flag = FALSE;

  while (!flag) {
    flag = doOneLetter();
  }
}  

int exitLetter (char c) {
  return ((c == 'Q') || (c == 'Z'));
}

Bool doOneLetter() {
  char l;
  int i;

  readLetter(l); // returns upper case letter
  if (exitLetter(l)) {
    cout << "Quit." << endl;
    return(TRUE);
  }
  else {
    i = computeCorrespondingDigit(l);
    cout << "The letter " << l << " corresponds to " << i 
      << " on the telephone" << endl;
    return(FALSE);
  }
}

int computeCorrespondingDigit (char l) {
  switch (l) {
  case 'A' : 
  case 'B' :
  case 'C' : 
    return(2);
  case 'D' : 
  case 'E' :
  case 'F' : 
    return(3);
  case 'G' : 
  case 'H' :
  case 'I' : 
    return(4);
  case 'J' : 
  case 'K' :
  case 'L' : 
    return(5);
  case 'M' : 
  case 'N' :
  case 'O' : 
    return(6);
  case 'P' : 
  case 'R' :
  case 'S' : 
    return(7);
  case 'T' : 
  case 'U' :
  case 'V' : 
    return(8);
  case 'W' : 
  case 'X' :
  case 'Y' : 
    return(9);
  }
}

void readLetter(char& c) {
  cout << "Enter a letter: ";
  cin >> c;
  while (cin && !isalpha(c)) {
    cout << "You have not entered a letter. Try again: ";
    cin >> c;
  }
  if (!cin) {
    cout << "Unrecoverable error... Exiting" << endl;
    exit(-1);
  }
  c = toupper(c);
}