protected function buildNationalNumberForParsing($numberToParse, &$nationalNumber)
    {
        $indexOfPhoneContext = strpos($numberToParse, static::RFC3966_PHONE_CONTEXT);
        if ($indexOfPhoneContext !== false) {
            $phoneContextStart = $indexOfPhoneContext + mb_strlen(static::RFC3966_PHONE_CONTEXT);
            // If the phone context contains a phone number prefix, we need to capture it, whereas domains
            // will be ignored.
            if ($phoneContextStart < (strlen($numberToParse) - 1)
                && substr($numberToParse, $phoneContextStart, 1) == static::PLUS_SIGN) {
                // Additional parameters might follow the phone context. If so, we will remove them here
                // because the parameters after phone context are not important for parsing the
                // phone number.
                $phoneContextEnd = strpos($numberToParse, ';', $phoneContextStart);
                if ($phoneContextEnd > 0) {
                    $nationalNumber .= substr($numberToParse, $phoneContextStart, $phoneContextEnd - $phoneContextStart);
                } else {
                    $nationalNumber .= substr($numberToParse, $phoneContextStart);
                }
            }

            // Now append everything between the "tel:" prefix and the phone-context. This should include
            // the national number, an optional extension or isdn-subaddress component. Note we also
            // handle the case when "tel:" is missing, as we have seen in some of the phone number inputs.
            // In that case, we append everything from the beginning.

            $indexOfRfc3966Prefix = strpos($numberToParse, static::RFC3966_PREFIX);
            $indexOfNationalNumber = ($indexOfRfc3966Prefix !== false) ? $indexOfRfc3966Prefix + strlen(static::RFC3966_PREFIX) : 0;
            $nationalNumber .= substr($numberToParse, $indexOfNationalNumber,
                $indexOfPhoneContext - $indexOfNationalNumber);
        } else {
            // Extract a possible number from the string passed in (this strips leading characters that
            // could not be the start of a phone number.)
            $nationalNumber .= static::extractPossibleNumber($numberToParse);
        }

        // Delete the isdn-subaddress and everything after it if it is present. Note extension won't
        // appear at the same time with isdn-subaddress according to paragraph 5.3 of the RFC3966 spec,
        $indexOfIsdn = strpos($nationalNumber, static::RFC3966_ISDN_SUBADDRESS);
        if ($indexOfIsdn > 0) {
            $nationalNumber = substr($nationalNumber, 0, $indexOfIsdn);
        }
        // If both phone context and isdn-subaddress are absent but other parameters are present, the
        // parameters are left in nationalNumber. This is because we are concerned about deleting
        // content from a potential number string when there is no strong evidence that the number is
        // actually written in RFC3966.
    }