libphonenumber\PhoneNumberUtil::getRegionCodesForCountryCode PHP Method

    public function getRegionCodesForCountryCode($countryCallingCode)
    {
        $regionCodes = isset($this->countryCallingCodeToRegionCodeMap[$countryCallingCode]) ? $this->countryCallingCodeToRegionCodeMap[$countryCallingCode] : null;
        return $regionCodes === null ? array() : $regionCodes;
    }

 public function testGetRegionCodesForCountryCode()
 {
     $regionCodesForNANPA = $this->phoneUtil->getRegionCodesForCountryCode(1);
     $this->assertContains(RegionCode::US, $regionCodesForNANPA);
     $this->assertContains(RegionCode::BS, $regionCodesForNANPA);
     $this->assertContains(RegionCode::GB, $this->phoneUtil->getRegionCodesForCountryCode(44));
     $this->assertContains(RegionCode::DE, $this->phoneUtil->getRegionCodesForCountryCode(49));
     $this->assertContains(RegionCode::UN001, $this->phoneUtil->getRegionCodesForCountryCode(800));
     // Test with invalid country calling code.
     $this->assertEmpty($this->phoneUtil->getRegionCodesForCountryCode(-1));
 }