超出范围异常 - 子串 - C＃(Out of Range Exception - Substring - C#)

 我有一些电话号码列表，其中一些有扩展名，有些则没有。 我正式使用我的电话号码只返回数字。 
 
 然后我为前10位数字（区号+数字）创建一个字符串。 
 
 我比较这个字符串长度并将其与原始字符串进行比较。 如果存在差异，我将余数用作扩展名。 
 
 在下面的代码中，我一直在超出范围异常。 我调试了它，它似乎没有超出范围。 有人看到我错过了吗？ 
 
var prePhone = emp.y.PhoneNumber;

if (!string.IsNullOrEmpty(prePhone))
{
    string xPhone = Regex.Replace(prePhone, "[^0-9]", "");
    string number = xPhone.Substring(0, 10);
    int extMath = xPhone.Length - number.Length;

    if (extMath >= 1)
    {   int preExt = 9 + extMath;
        string ext = xPhone.Substring(10, preExt);//Out of range exception
        em.Phone = beautifyPhoneNumber(number, ext);
    }
    else {
        string ext = null;
        em.Phone = beautifyPhoneNumber(number, ext);
    }
}

I have a list of phone numbers some with extensions, some with out. I regex my phone number to return just the digits.
 
I then create a string for the first 10 digits(area code + number).
 
I compare this strings length and compare it to the original. If there is a difference, I use the remainder as the extension.
 
In the code below, I keep getting an out of range exception. I've debugged it, and it does not appear to be out of range. Anyone see what I'm missing?
 
var prePhone = emp.y.PhoneNumber;

if (!string.IsNullOrEmpty(prePhone))
{
    string xPhone = Regex.Replace(prePhone, "[^0-9]", "");
    string number = xPhone.Substring(0, 10);
    int extMath = xPhone.Length - number.Length;

    if (extMath >= 1)
    {   int preExt = 9 + extMath;
        string ext = xPhone.Substring(10, preExt);//Out of range exception
        em.Phone = beautifyPhoneNumber(number, ext);
    }
    else {
        string ext = null;
        em.Phone = beautifyPhoneNumber(number, ext);
    }
}

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