在输入中需要一个前导+

如何更新此代码以使前置加号+字符必需?

<script type="text/javascript" language="javascript">
// Declaring required variables
var digits = "0123456789";
// non-digit characters which are allowed in phone numbers
var phoneNumberDelimiters = "()- ";
// characters which are allowed in international phone numbers
// (a leading + is OK)
var validWorldPhoneChars = phoneNumberDelimiters + "+";
// Minimum no of digits in an international phone no.
var minDigitsInIPhoneNumber = 10;
function isInteger(s)
{   var i;
    for (i = 0; i < s.length; i++)
    {   
        // Check that current character is number.
        var c = s.charAt(i);
        if (((c < "0") || (c > "9"))) return false;
    }
    // All characters are numbers.
    return true;
}
function trim(s)
{   var i;
    var returnString = "";
    // Search through string's characters one by one.
    // If character is not a whitespace, append to returnString.
    for (i = 0; i < s.length; i++)
    {   
        // Check that current character isn't whitespace.
        var c = s.charAt(i);
        if (c != " ") returnString += c;
    }
    return returnString;
}
function stripCharsInBag(s, bag)
{   var i;
    var returnString = "";
    // Search through string's characters one by one.
    // If character is not in bag, append to returnString.
    for (i = 0; i < s.length; i++)
    {   
        // Check that current character isn't whitespace.
        var c = s.charAt(i);
        if (bag.indexOf(c) == -1) returnString += c;
    }
    return returnString;
}
function checkInternationalPhone(strPhone){
    var bracket=3
    strPhone=trim(strPhone)
    if(strPhone.indexOf("+")>1) return false
    if(strPhone.indexOf("-")!=-1)bracket=bracket+1
    if(strPhone.indexOf("(")!=-1 && strPhone.indexOf("(")>bracket)return false
    var brchr=strPhone.indexOf("(")
    if(strPhone.indexOf("(")!=-1 && strPhone.charAt(brchr+2)!=")")return     false
    if(strPhone.indexOf("(")==-1 && strPhone.indexOf(")")!=-1)return false
    s=stripCharsInBag(strPhone,validWorldPhoneChars);
    return (isInteger(s) && s.length >= minDigitsInIPhoneNumber);
}
function ValidateForm(){
    var Phone=document.frm_cntctus.custom_phone
    if ((Phone.value==null)||(Phone.value=="")){
        alert("Please enter your phone number")
        Phone.focus()
        return false
    }
    if (checkInternationalPhone(Phone.value)==false){
        alert("Please enter a valid phone number.")
        Phone.value=""
        Phone.focus()
        return false
    }
    return true
 }
</script>