<script language="javascript">
    function chk_tel(str, field) {
        var str;
        str = checkDigit(str);
        len = str.length;

        if (len == 8) {
            if (str.substring(0, 2) == 02) {
                error_numbr(str, field);
            } else {
                field.value = phone_format(1, str);
            }
        } else if (len == 9) {
            if (str.substring(0, 2) == 02) {
                field.value = phone_format(2, str);
            } else {
                error_numbr(str, field);
            }
        } else if (len == 10) {
            if (str.substring(0, 2) == 02) {
                field.value = phone_format(2, str);
            } else {
                field.value = phone_format(3, str);
            }
        } else if (len == 11) {
            if (str.substring(0, 2) == 02) {
                error_numbr(str, field);
            } else {
                field.value = phone_format(3, str);
            }
        } else {
            error_numbr(str, field);
        }
    }

    function checkDigit(num) {
        var Digit = "1234567890";
        var string = num;
        var len = string.length
        var retVal = "";

        for (i = 0; i < len; i++) {
            if (Digit.indexOf(string.substring(i, i + 1)) >= 0) {
                retVal = retVal + string.substring(i, i + 1);
            }
        }
        return retVal;
    }

    function phone_format(type, num) {
        if (type == 1) {
            return num.replace(/([0-9]{4})([0-9]{4})/, "$1-$2");
        } else if (type == 2) {
            return num.replace(/([0-9]{2})([0-9]+)([0-9]{4})/, "$1-$2-$3");
        } else {
            return num.replace(/(^01.{1}|[0-9]{3})([0-9]+)([0-9]{4})/, "$1-$2-$3");
        }
    }

    function error_numbr(str, field) {
        alert("정상적인 번호가 아닙니다.");
        field.value = "";
        field.focus();
        return;
    }
</script>

<input type="input" name="tel1" onblur="chk_tel(this.value,this);">

사용예)