Phone Number Converter

#include <iostream>
#include <string>
#include <limits>

using namespace std;

void telephoneConverter();
void repeatInput();
int counter;
char choice;
char phoneNumber;
char repeat;

int main()

{

    cout << "Enter Y/y to convert a telephone number from letters to digits.";

    cout << endl
         << "Enter any other letter to terminate the program.";
    cin >> choice;

    if (choice == 'Y' || choice == 'y') {

        telephoneConverter();
        repeatInput();
    }

    else if (choice != 'Y' || choice != 'y') {

        repeatInput();
    }

    return 0;
}

void telephoneConverter()
{

    cout << "\nEnter a phone number in letters only."
         << endl;

    for (counter = 0; counter < 7; counter++) {
        cin >> phoneNumber;

        if (counter == 3)
            cout << "-";

        if ((phoneNumber >= 'A' && phoneNumber <= 'Z')
            || (phoneNumber >= 'a' && phoneNumber <= 'z'))
            switch (phoneNumber) {
            case 'A':
            case 'a':
            case 'B':
            case 'b':
            case 'C':
            case 'c':
                cout << 2;
                break;
            case 'D':
            case 'd':
            case 'E':
            case 'e':
            case 'F':
            case 'f':
                cout << 3;
                break;
            case 'G':
            case 'g':
            case 'H':
            case 'h':
            case 'I':
            case 'i':
                cout << 4;
                break;

            case 'J':
            case 'j':
            case 'K':
            case 'k':
            case 'L':
            case 'l':
                cout << 5;
                break;
            case 'M':
            case 'm':
            case 'N':
            case 'n':
            case 'O':
            case 'o':
                cout << 6;
                break;
            case 'P':
            case 'p':
            case 'Q':
            case 'q':
            case 'R':
            case 'r':
            case 'S':
            case 's':
                cout << 7;
                break;

            case 'T':
            case 't':
            case 'U':
            case 'u':
            case 'V':
            case 'v':
                cout << 8;
                break;

            case 'W':
            case 'w':
            case 'X':
            case 'x':
            case 'Y':
            case 'y':
            case 'Z':
            case 'z':
                cout << 9;
                break;
            }
    }
};

void repeatInput()
{
    cout << endl
         << "Would you like to enter a phone number: Y/N ";
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    cin >> repeat;
    if (repeat == 'Y' || repeat == 'y') {

        telephoneConverter();
        repeatInput();
    }
    if (repeat == 'N' || repeat == 'n') {
        cout << "Have a good day.";
    }
    else {
        cout << "Please enter a valid input: Y/N.";
        repeatInput();
    }
};