The solution to this problem is relatively simple: you have to iterate through all the phone numbers and copy to a separate container (such as an std::vector) the phone numbers that start with the country code. If the specified country code is, for instance, 44, then you must check for both 44 and +44. Filtering the input range in this manner is possible using the std::copy_if() function. A solution to this problem is shown here:

bool starts_with(std::string_view str, std::string_view prefix)
{
   return str.find(prefix) == 0;
}

template <typename InputIt>
std::vector<std::string> filter_numbers(InputIt begin, InputIt end,
                                        std::string const & countryCode)
{
   std::vector<std::string> result;
   std::copy_if(
      begin, end,
      std::back_inserter(result),
      [countryCode](auto const & number) {
         return starts_with(number, countryCode) ||
                starts_with(number, "+" + countryCode);
   });
   return result;
}

std::vector<std::string> filter_numbers(
   std::vector<std::string> const & numbers,
   std::string const & countryCode)
{
   return filter_numbers(std::cbegin(numbers), std::cend(numbers), 
                         countryCode);
}

This is how this function can be used:

int main()
{
   std::vector<std::string> numbers{
      "+40744909080",
      "44 7520 112233",
      "+44 7555 123456",
      "40 7200 123456",
      "7555 123456"
   };

   auto result = filter_numbers(numbers, "44");

   for (auto const & number : result)
   {
      std::cout << number << std::endl;
   }
}