无法将位数作为输入进行计数

我的程序：从用户那里获得有关个人详细信息的输入，然后输出信息。

我的问题：程序无法计算作为"电话号码"字段输入的位数。它接受超过10个数字作为电话号码的输入。

我的目标：检查输入的"电话号码"并确保该号码为10位数字。

我的代码：

#include <iostream>
#include <string>
#include <limits>
using namespace std;
class Personal_Details
{
public:
    void setFirstName   (string newFirstName);
    void setLastName    (string newLastName);
    void setBirthdate   (int newBirthday);
    void setEMail       (string newEMail);
    void setPhoneNumber (int newPhoneNumber);
    void QUESTIONS();
    string getFirstName();
    string getLastName();
    string getEMail();
    int getPhoneNumber();
    int    getBirthdate();
private:
    string FirstName;
    string LastName;
    string EMail;
    int PhoneNumber;
    int Birthdate;
};
void Personal_Details :: setFirstName (string newFirstName)
{
    FirstName = newFirstName;
}
void Personal_Details :: setLastName (string newLastName)
{
    LastName = newLastName;
}
void Personal_Details :: setBirthdate (int newBirthdate)
{
    Birthdate = newBirthdate;
}
void Personal_Details :: setEMail (string newEMail)
{
    EMail = newEMail;
}
void Personal_Details :: setPhoneNumber (int newPhoneNumber)
{
    PhoneNumber = newPhoneNumber;
}
string Personal_Details :: getFirstName()
{
    return FirstName;
}
string Personal_Details :: getLastName()
{
    return LastName;
}
int Personal_Details :: getBirthdate()
{
    return Birthdate;
}
string Personal_Details :: getEMail()
{
    return EMail;
}
int Personal_Details :: getPhoneNumber()
{
    if (PhoneNumber>12)
    {
        cout <<"INVALID. Program will now close.";
    }
    system ("PAUSE");
    return PhoneNumber;
}
void Personal_Details :: QUESTIONS()
{
    cout << "A.) Enter the following details:-" << endl <<endl;
    cout << "First Name: ";
    getline (cin, FirstName);
    cout << endl;
    cout << "Last (Family) Name: ";
    getline (cin, LastName);
    cout << endl;
    cout << "Birthdate: ";
    cin >> Birthdate;
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), 'n');
    cout << endl;
    cout << "Email Address: ";
    getline (cin, EMail);
    cout << endl;
    cout << "Phone Number: ";
    cin >> PhoneNumber;
    cout << endl;
    cout<< FirstName <<" "<< LastName << endl 
        <<"Birthdate: "<< Birthdate << endl 
        <<"Email ID: "<< EMail << endl 
        <<"Phone Number: "<< PhoneNumber << endl;
}
int main()
{
    Personal_Details NewContact;
    cout << "- Add New Contact Information -" << endl << endl; 
    cout << "Note: Please type in only intergers (numbers) for 'Birthdate' and 'Phone Number' field. Also, phone number should be less than/or equal to 10 digits." << endl;
    cout << "Else, this program will terminate abruptly." << endl << endl;
    NewContact.QUESTIONS();
    cout << endl << endl;
    cout << "Press Any Key to Exit.";
    cin.ignore();
    cin.get();
    return 0;
}

请你检查一下为什么我不能输入真正的10位数电话号码？我是C++的新手，所以我无法找到解决方案。谢谢你的帮助！