应该怎么修改让联系人的电话不重复？

应该怎么修改让联系人的电话不重复？
void addPerson(Addressbooks * abs)
{
//判断联系人是否满了
if (abs->m_Size == MAX)
{
cout << "通讯录已满，无法添加" << endl;
return;
}
else
{
cout << "请输入姓名：" << endl;
string name;
cin >> name;
abs->personArray[abs->m_Size].m_Name = name;

cout << "请输入邮政编码：" << endl;
int postcode;
cin >> postcode;
abs->personArray[abs->m_Size].m_Code = postcode;

string phone = "";
cout << "请输入联系电话：" << endl;
cin >> phone;

int ret = isExist(abs, phone);
    if (ret != -1)
    {
        cout << "通讯录中已经有姓名为"<<phone<<"的人,无法重复添加" << endl;
        return;
    }
abs->personArray[abs->m_Size].m_Phone = phone;

cout << "请输入家庭住址：" << endl;
string address;
cin >> address;
abs->personArray[abs->m_Size].m_Addr = address;

cout << "请输入E_mail：" << endl;
string e_mail;
cin >> e_mail;
abs->personArray[abs->m_Size].m_Mail = e_mail;

//更新通讯录人数
abs->m_Size++;

cout << "添加成功：" << endl;
system("pause");
system("cls");

}
}

int isExist(Addressbooks * abs, string phone)
{    
    for(int i=0;i<abs->m_Size;i++)
    {
        if(abs->personArray[i].m_Phone == phone) return 1;
    }
    return -1;    
}