This problem is somewhat similar in some aspects to the previous one. However, instead of selecting phone numbers that start with a specified country code, we must transform each number so that they all start with that country code preceded by a +. There are several cases that must be considered:

• The phone number starts with a 0. That indicates a number without a country code. To modify the number to include the country code we must replace the 0 with the actual country code, preceded by +.
• The phone number starts with the country code. In this case, we just prepend + sign to the beginning.
• The phone number starts with + followed by the country code. In this case, the number is already in the expected format.
• None of these cases applies, therefore the result is obtained by concatenating the country code preceded by + and the phone number. 

In all of the preceding cases, it is possible that the number could contain spaces. According to the requirements, these must be removed. The std::remove_if() and isspace() functions are used for this purpose.

The following is an implementation of the described solution:

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

void normalize_phone_numbers(std::vector<std::string>& numbers,
                             std::string const & countryCode)
{
   std::transform(
      std::cbegin(numbers), std::cend(numbers),
      std::begin(numbers),
      [countryCode](std::string const & number) {
         std::string result;
         if (number.size() > 0)
         {
            if (number[0] == '0')
               result = "+" + countryCode + 
                        number.substr(1);
            else if (starts_with(number, countryCode))
               result = "+" + number;
            else if (starts_with(number, "+" + countryCode))
               result = number;
            else
               result = "+" + countryCode + number;
      }

      result.erase(
         std::remove_if(std::begin(result), std::end(result),
            [](const char ch) {return isspace(ch); }),
         std::end(result));
      
      return result;
   });
}

The following program normalizes a given list of phone numbers according to the requirement and prints them on the console:

int main()
{
   std::vector<std::string> numbers{
      "07555 123456",
      "07555123456",
      "+44 7555 123456",
      "44 7555 123456",
      "7555 123456"
   };

   normalize_phone_numbers(numbers, "44");

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