Phone number validations made in C++ not very good but it works

phone.cpp

#include <iostream>
#include <string>
#include <iomanip>
using namespace std;

char d1, d2, d3, d4, d5, d6, d7, d8;

char digitConvertion(char d){
	char value = d;
	switch (toupper(d)){
		case 'A': case 'B': case 'C':
			value = '2';
		case 'D': case 'E': case 'F':
			value = '3';
		case 'G': case 'H': case 'I':
			value = '4';
		case 'J': case 'K': case 'L':
			value = '5';
		case 'M': case 'N': case 'O': 
			value = '6';
		case 'P': case 'Q': case 'R': case 'S':
			value = '7';
		case 'T': case 'U': case 'V':
			value = '8';
		case 'W': case 'X': case 'Y': case 'Z':
			value = '9';
	}
	return value;
}
bool isNumber(char& d){
	if(d == '1' || d == '2' || d == '3' || d == '4' || d == '5' || d =='6' || d == '7' || d == '8' || d == '9' || d == '0'){
		return true;
	} else {
		return false;
	}

}
void whichOne(){
	if(!isNumber(d1)){
		cout << "The first digit is a not a number" << endl;
	} 
	if (!isNumber(d2)){
		cout << "The Second digit is a not a number" << endl;
	} 
	if (!isNumber(d3)){
		cout << "The Third digit is a not a number" << endl;
	} 
	if (!isNumber(d5)) {
		cout << "The Fifth digit is a not a number" << endl;
	} 
	if(!isNumber(d6)){
		cout << "The Sixth digit is a not a number" << endl;
	}
	if (!isNumber(d7)){
		cout << "The Seventh digit is not a number" << endl;
	}
	if (!isNumber(d8)){
		cout << "The Ninth digit is not a number" << endl;
	}
}

bool validNumber(){
	if(isNumber(d1) && isNumber(d2) && isNumber(d3) && isNumber(d5) && isNumber(d6) && isNumber(d7) && isNumber(d8)){
		return true;
	} else {
		return false;
	}
}



void clean(){
	d1 = digitConvertion(d1);
	d2 = digitConvertion(d2);
	d3 = digitConvertion(d3);
	d5 = digitConvertion(d5);
	d6 = digitConvertion(d6);
	d7 = digitConvertion(d7);
	d8 = digitConvertion(d8);
}

string phoneToString(){
	string phone = "";
	phone += d1;
	phone += d2; 
	phone += d3; 
	phone += d4; 
	phone += d5; 
	phone += d6; 
	phone += d7;
	phone += d8;
	return phone;
}

string firstThree(){
	string three;
	three += d1;
	three += d2;
	three += d3;
	return three;
}


void main() {
	bool invalidPhone = true;
	string phone;
	cout << "Please type your phone number thanks you (Q to Quit): ";
	while(invalidPhone){
		cin >> d1 >> d2 >> d3 >> d4 >> d5 >> d6 >> d7 >> d8;
		if(d1 == 'Q'){
			invalidPhone = false;
		}
		clean();
		if(d4 != '-'){
			cout << "Your phone number should look like this 555-4444" << endl;
			cout << "However your phone looks like this " << phoneToString() << endl;
		} else if (d1 == '0') {
			cout << "Your phone number can't begin with a zero" << endl;
		} else if (firstThree() == "555"){
			cout << "Your phone number can't begin with 555" << endl;
		} else if (!validNumber()) {
			cout << "You used a invalid character please use a valid charcter" << endl;
			whichOne();
		} else {
			invalidPhone = false;			
		}
		if(invalidPhone == true){
			cout << "Please try again (Q to Quit): ";
		}
		
	}
	cout << "You have put in a valid phone number it is " << phoneToString() << endl;



	system("pause");

}