C++ Program to Convert English Words into Phone Numbers

#include <iostream.h>
#include<conio.h>
void main()
{
  char letter;
  int sum = 0;
  cout<<"enter digit :"<<endl;
while ( cin>>(letter) && sum < 7 ) {
 if (letter != ' ' && letter >= 'a' && letter <= 'z' && letter >= 'A' && letter <= 'z') sum++;
 if (letter > 'Z' && letter > 'z') {letter = (int)letter-32;}
 if(sum == 4) {cout << "-";}
 switch (letter) 
 {
   case 'a':
   case 'b':
   case 'c':
   cout << "2";
   break;
   case 'd':
   case 'e':
   case 'f':
   cout << "3";
   break;
   case 'g':
   case 'h':
   case 'i':
   cout << "4";
   break;
   case 'j':
   case 'k':
   case 'l':
   cout << "5";
   break;
   case 'm':
   case 'n':
   case 'o':
   cout << "6";
   break;
   case 'p':
   case 'q':
   case 'r':
   case 's':
   cout << "7";
   break;
   case 't':
   case 'u':
   case 'v':
   cout << "8";
   break;
   case 'w':
   case 'x':
   case 'y':
   case 'z':
   cout << "9";
   cout<<"\n";
   break;
   case 'A':
   case 'B':
   case 'C':
   cout << "2";
   break;
   case 'D':
   case 'E':
   case 'F':
   cout << "3";
   break;
   case 'G':
   case 'H':
   case 'I':
   cout << "4";
   break;
   case 'J':
   case 'K':
   case 'L':
   cout << "5";
   break;
   case 'M':
   case 'N':
   case 'O':
   cout << "6";
   break;
   case 'P':
   case 'Q':
   case 'R':
   case 'S':
   cout << "7";
   break;
   case 'T':
   case 'U':
   case 'V':
   cout << "8";
   break;
   case 'W':
   case 'X':
   case 'Y':
   case 'Z':
   cout << "9";
   cout<<"\n";
   break;
   default:
   cout << "Invalid input" << endl;
   getch();
}
}
}